The problem with focusing on constant self-optimization is that it is a process without end. We can never say we've reached the full version of ourselves. We may feel that we are not (and can never be) good enough. We're never allowed to be happy and satisfied.
Also, it's not that you shouldn't become your best self, but the concern is that the self-help craze, with the drive to optimize yourself all the time, has become pathological.