Why does my ex treats her better than me?

My ex and I broke up for 1.5 year. He got into a new relationship and i am still single. I thought i am ready to be friends with him again so i decided to meet up with him. We had good conversation and after that i went home. I came home and started to stalk his girlfriend social media. I got triggered because he treated her so much better than me. They went to Europe together and looked happy. ( we were in college when we were together) Now i feel life is unfair and how come i never had this kind of treatment.
Why does my ex treats her better than me?
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