Mostly people falling victim to commercialism and the fact that for some reason, it is one of the few things ALL societies can come to accept regardless of background. People only “love” each other because other people “love” each other. Love is pretty much a disease. Why do you think they call it lovesick.

As for what Shii said. Log in to see images!

That may also be it.

It is simply human urge to repopulate, as with most organisms.