I always trust people, to a certain extent, until they give me a reason not to.
That's my philosophy also. Nobody really knows anyone, in my opinion. This is because we are constantly changing and re-inventing our inner-selves. I am not the same person I was a year ago, or two years ago, or...well, you catch my drift. All I can do is give people the benefit of the doubt, and if they surprise me with their actions then I ruminate over what it means and whether or not I have to re-evaluate my relationship with that person.