1. You've been hurt before and you're afraid it will happen again.
2. You make yourself exposed. Like giving the keys to your house, making the other a cosignatory to your account, sharing secrets somebody else can blackmail you with, leaving your girlfriend to the care of your buddy, etc.
3. Trust is impossible to earn. Come to think of it, by what standards do we measure someone who we want to trust. How do we know if it's time to trust someone? And is it solely through time? If so, what length of time is required? The answer could only be arbitrary. A father might not trust his son with the car; a brother his secrets with his sister; a friend can show his true colors only after decades; and the wife can learn about her husband's transgressions after the fifth child. Perhaps trust is given until further notice, making it for the meantime, momentary, "for now I trust you, maybe later no longer"--or again arbitrary. If trust is momentary and arbitrary--can we still call this trust? Or is this not just being blind and naive?
4. You look for the bad in people.
5. You wouldn't even trust you.