I don't know whether you can trust your best friend or not; I don't know him or her.
The real question is: How do you feel about yourself? We judge and evaluate the world from our own character. If we're honest, then we regard most others as honest. If we're coming from a criminal nature, then the rest of the world is hostile and it seems to us that it cannot be trusted. Would you take advantage of other people? Then you're of the belief that they'll do the same to you and that no one can be trusted.
If you've been brought up in a trustworthy, loving environment you're way ahead. If you haven't, you need to take action to shed your negativity and beliefs and take on new and happier ones.