Why did he come back after he chose someone else?

Hello all.
I was talking to a guy for several months. We hungout once and went hiking, but we texted a lot and went to bars together a lot. He seemed like he really liked me... we got into one argument, stopped talking, and then he snapchatted me off someone elses phone to add him back onto snapchat. So i did. after a while he started dating another woman. They became an exclusive couple. A couple days ago he snapchatted me out of nowhere. I asked why, he said "we can't be friends?" . He still has the girlfriend. They also went hiking a few days ago. Any thoughts on why he randomly reached out? We were super into each other and then drifted apart.
Why did he come back after he chose someone else?
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