Reciprocation?

Why do we become friends with the people that we become friends with?  Why do we make sacrifices for the people that we make sacrifices for?  Why do we continue to make sacrifices for those that we know would never reciprocate for us?  Is it wrong to ask for reciprocation in a relationship?  Should we always start out with the premise that we should give without seeking return?  Can such a relationship last?  What should friendship be like?  Why do we bother investing in someone that we may not ever see again after a few months?