Why?

Sometimes we wonder why certain things happen. Truth be told I have. But one think I've caught myself tripping over is how.
How a friend supposedly could hurt someone so badly that they questioned their whole intire friendship.

I liked him.
Even loved him.
And every word I say about him is true.
How I believed he was so perfect, and that just seeing him would make my day and cause me to smile all the time. How he was so funny and beautiful and nothing could stop me liking him and no matter what I would always choose him.
Sounds cringey I know. Trust me, I've caught myself thinking that aswell. But it was true or still is im not sure which one. It could still be true as I always think about it to myself, silently.
He was everything anyone could care for and why would someone not like him because it was amazing in every single way possible and nothing he could do would stop me liking him. And then when your told that he doesnt like you back and you just die a little bit. Thats what happened to me several times throughout my friendship with this certain boy.
And how you told your friends this and you could tell they thought you were a little bit mad. But you didnt care because to you it made sense.
Then friends kissed him and yes friends, plural...
More than one, I just couldnt believe it and the fact for one of the girls I was in the party at the time. I shouldnt be friends with this girl but I decided to think about it like someone who could let things go and not hold grudges.
But the funny thing is, this happened over a year ago. I hold grudges and big ones I just dont act upon it. But when I see someone they used to see, I get abused how does this work..
Why does someone feel the need to hurt their friend, and with that certain boy? How?
I guess, I'll never know what went through her head when she did what she did. But I still find myself questioning something i have been for a while and still am.. Why am I friends with you again? Why? How?