Why am I bothered by my ex’s engagement?

My ex had a friend that he met after he got with me. I found the friendship suspicious being that he has a particular type and she fits it. He said that he wasn’t attracted to her and he was just friends with her to be nice. He claimed that she was obsessed with him and wouldn’t leave him alone. After we break up due to unrelated reasons (things just got a bit toxic), he gets with this very same woman that he claimed he wasn’t attracted to and now they are engaged less than a year later.

Although, I ended things for some reason I’m bothered by this.
Why am I bothered by my ex’s engagement?
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