It isn't easy to start having faith in people especially if they've hurt or betrayed you in the past. You might forgive them but you may find it very hard to trust them again. I suppose though if you really want to have faith in them you'd need to embrace the golden rule. Do to unto others what you expect them to do unto you. Whether we use drugs or not, we all make mistakes. You'd expect someone you've wronged to forgive you and probably trust you if you apologize and try your best to earn their trust once again. Why don't you do the same thing for them?
I know what you mean. I think the thing is that we have this sick culture of negativity. A lot of our values are questionable. Media bombards us with negativity and negative spins on things, even when there are positive stories or positive examples to give as well. No wonder people find it hard to be inherently positive. I find that if you get in the habit of it, though, rewire your brain so to speak, you'll see more positive behavior than you would have expected.