Taking a reasonable step back and making fair judgements about yourself is important - sometimes, you've got to listen to what your instincts are telling you, rather than just drown them out. But it's also very easy to get stuck in negative patterns of thought which actually prevent improvement and progress toward happiness. That's where programmes of self-correction have been shown to have significant measurable benefits.

Also, life is a codified self-delusion. You self-delude every single day in order to not go insane in the context of a society. CBT is merely acknowledging and exercising a little more control over the things we tell ourselves.