on relationships and expectations

Expecting things from a close friend comes so naturally. We get attached to someone for some reason and start creating expectations. But why?

If I consider someone very close to me, it doesn't mean that the other person also considers me equally close to him or her. In fact, being a different person, their definition of closeness itself might be totally different.

Why do I still expect them to respond for certain things? And when they don't, why do I get disappointed?

Is this the nature of relationships? Can I still consider someone a really close friend even if they don't? Can I simply keep loving, without expecting love in return? Is love, after all, such a selfish concept that one loves only for getting love back?