Ultimately, its all about passing DNA on.
If I love my wife, I've found someone with whom I can reproduce effectively. If I continue to love her, then I've found someone with whom I can share the responsibility of parenthood and raise a child to adulthood.
If I love my daughter, I'm going to be protective of her because I'll want the best possible partner for her when she's an adult and she can pass on my DNA. Hopefully the best parts.
If I love my neighbor, I'm positioning my self for favor in the future -politicking so that I'm looked upon favorably and, thus, adding another "friend" to my social network. Friends are beneficial in times of need (large needs and small needs).
And so on.
All this occurs behind the scenes, under the guise of "love," but all with the ultimate aim of reproducing and raising/protecting that reproduction; positioning that reproduction for the best possible future.
Or, at least, its my educated opinion.