there is probably not a straight answer.
I guess there are many ways of thinking about it. If you think of it in terms of cold logic, there would probably be no reasons whatsoever apart from self interest. But then, nobody says we need to be logic all the time.
You could also think in terms of evolution. In that case love might be seen as a mechanism to make you protect your close relatives.
And you could go on and on about all the teories. There is still research going on in philosophy departments about this kind of thing, and I don't think we will have a definite solution.
Whatever the reasons are, you still have the feelings, and you can cultivate them (through metta practices, nlp or whatever) or 'reduce' them.