Why can't people just stop being selfish?

It's a Friday today. Last night, a friend invited me for a jog and i agreed. But being me there was one problem. Approval from my parents. I was sick of being couped up in my own house, so I took the plunge this time. I lied to my mom. I told her that there wasn't school tomorrow but I had to go since I had Scouts meeting. She said fine. And so I went to bed that night guilty but excited. I woke up at the crack of dawn, brushed my teeth, took my shower, ate my breakfast with a plain white shirt on and my half wet towel wrapped around my waist. I had a cup of coffee and half a cracker. I walked to school with my brother and met Shien shin at the pentas out of school. She was wearing a black polo-T along with a pair of black shorts. The dark color of her attire complemented the color of her skin as she was reading a book. We talked a bit, and waited for the rest of the gang. When Wei See, Kok Hong and another dude who's name i don;t know came we left. We walked to FRIM, then was stopped at the entrance counter. We were asked to pay RM 1.00 as entrance fee. I don't know about the others but I sulked.....I honestly didn't want to pay, but had to. Then we continued. It was a nice slow nature walk. I felt like the burdens on my shoulders were slowly disappearing. It was really peaceful. Until Wei See started to pick us the pace, she started jogging, and the rest of us played 'follow the leader'. We jogged till the junction and took a right turn to head to the waterfall. We didn't go all the way. I was a little disappointed, Kok Hong asked if I wanted to go to the tennis court and I said yes without much thought. We were amateurs playing tennis. While the boys were playing I chatted with Shien Shin. It was the first time I saw her talk to me like that. She was emotionally hurt and I could do nothing. I felt angry. At my helplessness and the person who caused her this pain. MY question is why do people have to be selfish? If the person you love already has someone else, just be happy for them and live with it instead of hurting them. Just because you love A and A loves B, it doesn't mean you should hate or be mad at B. B loves A for the same reason you do.