Why has my ex done this?

We were amazing together, things ended not on great terms. We work in a bar... There was this girl I knew, 16 at the time but she still came out with a fake id. My ex is now with her; she's 18 and nothing that he'd normally go for... As he has a strict type, but I still hear from him with 'stop being cute' and your looking lovely today' ... But al along with out me realising he's been with her; but he have been over a while; but it hurts so much still. He's 25 as well and I'm 21
Why has my ex done this?
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