On the bus, an ex-friend sat in my place. I pick a new spot. A girl then tells me to leave rudely. I calmly say I can sit where I want and made room for her. The driver stops her and she quits. Why do I feel bad? Why do I hate my ex-friend?

Normally, seats on the bus are not assigned. Therefore, she didn't sit in your seat. She sat down where she wanted, just as you're free to do. A mature person would say "HI" and find a new seat on the bus. An immature child would whine and stomp their feet and want someone else to move.