Because you can only write so many songs about pick up trucks and your dog. Main reason is everyone relates to songs about love.
I am speaking of general songs...not crap rap... All of us are emotional I guess that's why.. BTW, I really love this.. http://youtube.com/watch?v=jhiss9goIaE
It's an emotion, one that drives almost everyone and brings up all sorts of feelings. People can relate.
Because, people post songs about things they are passionate about - and so they sing about their girlfriends etc.