I don't trust anybody, really, not for any bitterness on my part though. My reasoning is, and perhaps Mr/Miss MusicLover82 could relate, that I only have two extremes. I can either be extremely gullible or extremely skeptical. I feel that I have to err on the side of caution and assume others are untrustworthy otherwise I won't be able to tell whether they are tricking me or not. It's not very fun when you finally realize people you thought were being nice were actually being rude.
Even if you are good at noticing, I would advise against trusting anyone completely simply because you can never know anyone but yourself. You don't even really know your own parents, as they are the only people with access to their thoughts and feelings.