Why did he leave me for someone else?

My now ex left me and like a week or two he got with someone else. It's like our 5 years together meant nothing. During the last days of our relationship he was quiet a couple days. Then he up and broke it off with me, nothing bad was even going on in our relationship. We even went on a vacation. I saw that his new girlfriend posts YouTube videos of them going out of town a lot, more than we ever did. It seems like he's treating her way better than me. I've done so much for him and I don't understand why he would do that to me. It really hurts. I was a good girlfriend to him, never really argued, wasn't problematic, just a chill person. He texted me a couple times after being with her but I just don't want to talk to him ever again.
Why did he leave me for someone else?
Add Opinion