Why do I always assume the worst from people

So I gave my manager a letter complaining about all the cats that were in the backyard and how they were peeing everywhere and taking the wood off from around my back door and he handed me an envelope with a letter in it just now. I thought it was gonna say something like "You're being evicted" or something but it was actually a letter thanking me for addressing the issue and letting me know that they're doing everything they can. I was actually happy that they took my words into account. Why do I always assume the worst from people? I isolate myself from people so much, I guess I get stuck in my own head. Things usually turn out to be the opposite of the negative things I expect from them...I guess I should give people some credit once in a while.