A lesson from my life: don't trust anyone.
You may as well assume that everybody is out to use you. That way, you'll at least be pleasantly surprised when you encounter the few people who are either not interested in using you, or who may (can you imagine!?) even possibly be interested in helping you. You will find that most connections will end once a person no longer has use for you or interest in you. I, myself, am guilty of letting connections wither once the benefit no longer outweighed the effort (yes, I'm a bad, bad person).
People are inherently opportunistic and parasitic. To the extent that you happen to factor into their agenda, you may become their friend for a while. We may as well start putting our signatures on "friendship contracts", since this is the unspoken truth of it all. Because when it comes to relationships, there is scant anything that is unconditional.