Well, just because there isn't much written about love before XI century doesn't mean that love is a construct itself. Simply there was no point to write about romances before then - we should consider literature evolution. Back to the past, as spyman said, a lot of writers (not only poets) wrote about love, and in some cases it's pretty much the conception of love we are used to (see Virgil, for example).
But as the conception of love varies in time, we cannot define "love". We can look for "that feature" being present in ALL the affective relationship in history, so linking brothership, friendship, adult or young love, and find a common point, which is then enriched by the factual situation (considering all the possible aspects depending on environments and times). If we do that we are not dealing with love in its usual conception, but how else could we deal with it?
Then, obviously there's the chemical process and the rest, but this seems to me to be the second step: something has to start the process, before.