Why would someone cheat with their ex?

My ex-boyfriend cheated on me with his ex-girlfriend. I don't feel like I was just a rebound until he got his ex back. It's not like he dumped me when she wanted him back. He could have had her if he wanted her. Instead, he cheated on me with her several times until I found out and left him. But what's the point of cheating with an ex? If he loved her so much and she loved him, why did they even break up to begin with?

Why would someone cheat with their ex?
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