He may think he's doing his good friend Stevie a favor.
She thinks she's doing him a favor, giving him something big to do that keeps him busy and not sinking into any sort of depression over Tom.
He gets to make a bunch of money (his songwriting royalties can't be bad either) and as he noted, he gets to travel around in private jets playing in big arenas.
Probably both interpretations have some validity. But they are rationalizations after the fact.
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