What is the reason why my friend didn't want to be my friend?

This happens a few years ago. When i was in primary school, i had a best friend called Samantha. Everything was alright until another girl came. So, she befriended Samantha and sort of steal my friend(Samantha) from me. One morning, during recess, i came and find Samantha and during that time, she was with her 2 friends in which 1 of them i knew(Jolyn). Basically, she was talking to them and when i came, the other 2 friends started laughing. I was curious at what they were laughing at but i didn't go and ask them. After school, while i was waiting for my remedial to start, Jolyn came to me and told me the reason why they were laughing that morning. She said that Samantha told them that she didn't want to be my friend anymore. At that time, i was sad and angry too thus i said: actually, i didn't want to be her friend too. We didn't talk and ignore each other from that day onwards and until now, i still don't know the reason why she react this way and why she didn't want to be my friend. Can someone tell me a possible reason? Thanks :)