They say that expectations kill relationships. But you can't have a relationship without expectations, can you? The same holds true for friendships, and that's one of the things that distinguishes friends from acquaintances.
I expect someone I consider a friend to be there for me when I need him/her, and I would do the same for them. But 31 years and I still err when it comes to separating my friends from acquaintances.
The key is to remember that there IS a reason why you aren't "friends" with someone you may have casually known for some time. People who're meant to be friends click at the first go. If you didn't - or it didn't occur to you both to take things to the next level - it's a sign that you should invest your emotions, time and energy elsewhere. If you pursue something that's not meant to be, you WILL be disappointed sooner rather than later.
I'm very lucky and blessed to have the friends that I do - friends who are there for me when I'm going through a rough time, who stick by me through thick or thin, who lift my spirits when I'm low, and who know when they need to put me before themselves because my need at that time may be greater than theirs. I try to be there for my friends as much as I can. And that is how the cookie crumbles.