Why is he all over sudden interested when I started to see someone else?

I was in this situationship with this guy for about 4 months. We hung out, had sex, I met his friends, we had lots of fun together but I could tell he didn’t want anything serious. Initially, I didn’t want anything serious either but later, I started to like him (I’m stupid, I know). Then I stopped talking to me for like two weeks, and would only respond with short replies.
so I stopped messaging/calling and I moved on and I have started to date someone else who I really like because his simply amazing!

Now my situationship keeps messaging and checking up on me even more than before. I don’t think he wants anything serious, why can’t he let me be happy?

Why is he all over sudden interested when I started to see someone else?
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