Why do we love?

I was thinking about this on the loo a few minutes ago.
On the cubicle door there was so much graffiti- (not in my house, btw) Stuff saying 'E.J for R.C forever', and 'I love david hopkins' etc.

I was thinking about, of all the tings we carry around with us inside our heads, why is love such a big deal? Why does no-one write 'I love my job' or 'Bethany is my best friend?'

Whats more- why do we love? I was asking my boyfriend this morning, 'Why do you love me?' And he couldn't really tell me, likewise, I couldn't tell him either. The only things I could think of was: that we get on so well together, that I think he is hot, that we always manage to sort out or arguments, we like doing the same things, we laugh and have loads of fun etc. etc.
There was never one underlying reason, and it kind of felt like we don't have a proper reason to go out, apart from the fact I think the world of him, and all I want is to see him happy and if I tried to seperate myself from him then I couldn't.