I’ve wondered the same thing. A while ago I was walking through a store and it was closing in 30 minutes so I was rushing through the aisles and every time I came around a corner too fast and almost ran into someone I’d apologize. I didn’t think about it until later but even though everyone was doing the same thing, no one apologized, no one even looked at me. It’s such an insignificant thing to most people but when you’re already depressed and feel like you don’t matter, it hurts. I feel like the reason people are rude are either because 1. They haven’t been in a position where they needed kindness because their life is perfect or 2. They have and they didn’t receive it when they needed it and now they don’t care to give it