As I finish my last year in college I can look back and see all that I have learned. Not only did I learn subject material from my classes but I also learned a lot about myself, others, and how the real world works.
I trusted easily, believed in the best in people too readily, hoped too much and was too forgiving. These qualities are all great to have when invested wisely. It's amazing to have people in your life to trust and believe in that you have high hopes for and forgive when transgressions inevitably happen (we are only human, after all)--but they need to be worth the emotional investment. These people need to invest back in you, trust you, believe in you, want the best in life for you and forgive your flaws as well. However, it's not always a two-way street.
That's okay, though, that's life. We aren't meant to keep every friend we make, sometimes their chapter in your life is done. That's okay. You both grew and learned something from it, and the relationship had an expiration date because of growing different directions, too different of lifestyles, or general busyness. Sometimes these same people will end up showing up in later chapters of your life as a reoccurring character. That reunion could be pretending you don't see each other in a grocery store or it can be someone you just stay Facebook friends with even though you haven't talked in years. These returners can rekindle into friendships, sometimes stronger than before because of the right proximity or overall maturity.
Some people stay in our lives forever but in waves, yet some are rocks in our lives like a sibling or parent (who can be friends in addition to being family).
You have to find out people's true colors somehow and to find the gold you have to pan through the bleak water. Discovering rocks in your life, the people who come in welcomed waves since life is busy, and the surprise of an appreciated recurrance of characters make putting up with loss worth it. That's life, and that's what keeps it interesting.