Why is my ex who never wanted a relationship acting weirdly now that he’s engaged?

I casually dated this guy 4 years ago for 6 months. I was moving in 1-2 years and we didn't actually have much in common, so it didn't make sense to be in a relationship. Of course, I still caught feelings eventually and he didn't want more so I had to end it. He cried, but it was an otherwise amicable breakup. Several months later, I bumped into him at a party since it turned out we had mutual friends. By then, I had moved on and he just started dating this woman who, from his description, seemed like the perfect match for him. It seemed we could become friends, so we started to grab a meal every now and then (after I moved, it became whenever one of us was in town) and text each other happy birthday.

It's been 2 years of this friendship and I've gradually noticed strange behavior:

  • Unless I directly ask, he'll never bring up his partner even in passing
  • He's interested in my love life and has even gotten uncomfortable when I've seriously dated anyone
  • He's said/done a couple things that felt off in the moment. They weren't explicitly flirty, so I thought I was overthinking and brushed it off. But in retrospect, those weren't things he'd say/do to a friend

It finally clicked for me when I met his partner, now fiancee, at a friend's wedding and she didn't seem to recognize me at all. I don't think he's ever told her about me. I also noticed him giving me intense looks throughout the night. Obviously, I'm not going to be friends with him anymore, but I'm confused because I don't think he's actually trying to cheat. He'd make more of a move and talk to me way more than he currently does if that was the case. But if he's not trying to cheat, what's his goal? Especially since he's engaged to the perfect woman for him and he didn't want anything serious with me? I feel like it can't just be because he's attracted to me, because he can be attracted to me but not act in this manner.

Why is my ex who never wanted a relationship acting weirdly now that he’s engaged?
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