Why did he always think he could do better even tho I was the best he’d been with?

During his first relationship age 12-17 he never tried with others well apparently he did but this is an ex saying he cheated.

He lacked confidence in getting people at this point massively

Then he had a one night stand then met me at 18-25 he always thought he could do better. even though his friends, family, all mines etc all said I was the best by far he’d had.

He always thought why am I with her when I could have this or that.

Why is this?
Why did he always think he could do better even tho I was the best he’d been with?
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