"Societal structures, personal interest, definitive moral good?"
In terms of how people actually DO make decisions in practice, I would suspect that all three play a role (sometimes singly, sometimes in combination), and the ones that actively come into play for a given person in a given situation depend on the person and the situation.

In descending order of importance of what (in my view) SHOULD be the reasons for making a decision on what is the right thing to do:
1) definitive moral good
2) societal structures
3) personal interest
But important to remember that, even on a "should" basis, there are (plenty of) circumstances in which there is no definitive moral good in play, and there are other times as well when there is no societal structure in place either.

Personally I go about situations as if I don't feel terrible after or I don't hesitate than it is right. If someone comes in my house with a gun, threatens my family,and I bash his skull in with a brick and I never hesitate and I don't feel bad after then to me it wasn't morally wrong. Like wise if I think about taking a dollar out of grannys purse and my stomach clenches and I hesitate than maybe I shouldn't do it. Now this doesn't work for everyone as some people lack empathy and other things I assume I have. But for me I use my gut, this means I am willing to trip the guy in front of me in the race if that race is for my survival or the survival of those I love.

Morals, in my opinion, can come from different sources depending on the person. For the strong-willed, such as myself, morals are an intrinsic force. For others, environmental factors and societal pressure play a greater role.

I believe there's always a 'best' or 'most right' thing to do and I try to do it.

Since I'm not a christian and ignore the standards and structures of society, it's not immediately obvious what I should try to achieve, which I believe is crucial: you can't find the best path to achieving a goal if you don't know what the goal is.

"If you don't know to what port you're sailing, no wind is favourable."
-Paraphrasing some Roman dude

As such, I have a pretty clear list of priorities in my head with a general theme of 'helping people' dominating the whole list. Interestingly, most people are unaware of my motives, even though they play a role in every decision I make.

Doing business with someone? If it's a powerful multinational, I'm going to demand almost their entire margin (which I usually can if I have something unique to offer), and that's assuming they aren't the kind of companies which pollute and exploit and what else..
If it's a start-up, I may still demand something, but not even necessarily if I believe someone is doing the right thing.
If it's a charity I support, I'm likely to ask nothing or, depending on what I can afford, only ask what I need to cover my costs.
My reasoning being that the only reason I care about money to begin with is so I can use it to help people, so receiving money from organisations who help people (or from people who need help with financing for that matter) is often counter-productive due to taxes, for instance.

School? Learning fundamental things which can potentially help me help others somehow.

In short, I have few morals, but I strive to do whatever is best to achieve my goals which works great for me. It gives everything a 'natural value' without having to think about it. I think saying you'll NEVER do something NO MATTER WHAT is naive because one can usually think of a situation where doing this thing you refuse to do is better for the world. It should always be looking at what's best to do in my opinion, typically measuring in (expected) results.

Does it help more than it hurts? Go on until you find something that helps even more.
Otherwise, go do something else. Simple.

I think Atticus Finch said it best, "before I can live with other folks I've got to live with myself. The one thing that doesn't abide by majority rule is a person's conscience."

Morality is important because people are important.

really? humans are important in an indefinably large universe? Isn't that statement in itself a social position?

I assume Flumnator means important to each other - not important to a bug-blatter beast from another dimension...

again a social position. Nothing wrong with that, just pointing it out.

But then, why wouldn't morality be anything but a social position? after all it specifically deals with how humans interact with the world around them, an intrinsically social phenomena.

^phenomenon

^an intrinsically obnoxious phenomenon.

Morality is important to me because it describes the kind of world i would orefer to live in (selfish/personal).

The kind of world i would prefer is dependent on my lived experiences, where i grew for (for example) determines my preference for gun control, who i have shared homes with determines my preference for living with other people or alone or in a commune, and the adventures i've gone on determines my preferences for wild camping/freedom from the system. This is all socially constructed.

My lack of religion and my philosophical outlook leaves with a strong sense that there can be no ovjective meaning, no 'definitive' morality, though not a nihilist - ibelieve that the only meaning in life is that which you make for yourself - i do believe in a God of sorts. I just don't know how i can understand the will of that god in any meaningful sense to better serve it and be less selfish. (My God is that of Spinoza, a Pantheist idea where Universe and God are one and the same, we are in essence aspects of the Universe trying to experience itself... I don't know how you can build a definitive morality from that fact, even deep study of a thing like entropy doesn't seem to help.)

So i am left with the best world i want to live in. (Just as i argue for the best webdip forum i want to engage in... Even if i also accept that there are others who prefer a different forum; i don't think they are wrong for having different preference to mine.)

The question was "Why Does Morality Matter?". Yes it does. I do not wish to socialise, meet or do any kind of business with people that are immoral, people that do lie to and you cannot trust. Moral to me is to behave to others as you wish them to act to yourself and the absence of double standards.

We're primates. If we were lizards, it wouldn't. Social animals though have a lot of behaviors that are cued to social relations, and a lot of the behaviors we find appealing are what we call morality. Of course, there's a strong cultural element to that, so there it matters because we were raised to think it mattered as well. (e.g., "morality" is very different place to place and time to time.)

Hmm I sort of doubt the lizards-statement.

Then you should check out the facebook page 'lizard people of new york' them lizards re whack, yo.

It is true that were are biologically pre-disposed towards being social, feeling part of a group is good for most of is (maybe sociopaths/psychopaths are an exception, and only follow the rules because it would hurt them not to) we are fairly unique among animals in that we can live (perhaps even thrive) in cities of millions. While not being related to those millions - (bees and other 'social' insects may compare in pure number living an co-operating, but they are normally more closely related, and othe animals may live socially with unrelated friends - i imagine dolphins, elephants and great apes - but they don't manage millions of individuals)

And for that a common sense of morality is necessary. You can see how important it is when you talk about something like toulet access for trans people, the debate is a struggle to come to some common sense of what is right. People think this is important because it is how we build the social rules of our society. (Not the legal rules, which are objects of institutional power, but the social rules which exist regardless of law - and infact preceed law in most cases.)

In theory it's nice to think that it matters but in practice it does not seem to matter much at all. Whatever the rules for "right action" are (and there at least seems to be a general idea of the content of these rules), they seem to be ignored with tremendous regularity.

"In theory there is no difference between theory and practice. In practice there is."

They're also obeyed with tremendous regularity, Putin33.

Morality comes from who you surround yourself with too. If you hang out with toxic people for example it can really make people become immoral quickly.

Drunks hang out with other drunks.. whereas
Evangelicals rarely spend time with drunks and druggies.

So what's their excuse then?

Also, Jesus allegedly hung around a lot of lumpenproletarians. Whenever I drive around the decayed sectors of the urban core I see a lot of liquor stores, payday loans, and churches. They seem to go together.