I can't trust others any more?

It looks like many people just don't care... and it really bothers me and hurts me. Maybe because I'm in UC Berkeley Engineering department where all the smart nerds are at. It's fine that they can be busy. But it really bothers me when my friends just leave class without telling me ahead that they are busy. It makes me assume they are angry at me if they do it continuously. And asking them to letting me know makes them angry somehow (as one of them called me "insecure") or saying at least hi or bye when you see your friend in the elevator. I've had a broken heart recently for similar reasons. I thought he doesn't like me since he didn't talk to me for two weeks(though I tried to talk to him..). Then he comes to me like nothing happened while I was getting over him, saying that he was busy with his friends hanging out. Fine that he's got his friends, but if and only if he let me know it ahead. I feel like he was playing games with me. Or other friends ignoring my texts all the time. Then they text me when they need me like nothing happened between us. It's fine that they are busy/selfish/or whatever, but I just feel like they don't have basic manners/politeness. They only care when they are related. And they don't know what they did wrong. They just don't seem to care, and I can't trust people any more. Hopefully when I grow up and graduate, not everyone is like that? I'm kind of sensitive, and I care about others. So it hasn't been easy for me to see and understand why they lack of basic communication/respect skills. You may think I'm overthinking and caring way too much, which could be right. But the fact that they just don't care others really bothers me... I graduated a christian high school, and it used to be good and better than now. Ever since I graduated my high school, this is the first impression of the real world... Hopefully one day somebody can prove not everyone's like that and I can trust people again. What do you guys think? Can I change my perspective and maybe think differently?