Why would a guy tell me he isn't interested in getting into a relationship, but days later, he is talking to another girl?

This guy told me he wasn't interested in getting into a relationship because I think he knew what my intentions were, but days later, I saw him with another girl, and they have been talking for days, and I always see them walking and eating lunch together. I am so mad at him for lying to me, and rejected me like that. I wish I could beat him up, and stomp on his face.

Why would a guy tell me he isn't interested in getting into a relationship, but days later, he is talking to another girl?
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