He says

The common idea that success spoils people by making them vain, egoistic and self-complacent, is erroneous; on  the contrary it makes them for most part, humble, tolerant and kind. Failure makes people cruel and bitter 

Failure inhibits you in two ways. It inhibits your ability to use the skill you've got, and it inhibits your ability to get better. You underachieve and you are stuck underachieving. (1978, p.58)