To be truly successful (e.g. not just professionally, but personally, and as part of the community), a person must know themself.
Happiness comes when your self-perception and the outside world line up.
When a person who has a high self-opinion is seen by the outside world as a failure, the result is anger and resentment.
When a person who has a low self-opinion is seen by the outside world as a success, the result is uneasiness and a fear of being unmasked.
As long as you fail to know yourself, your secret insecurities will sabotage your happiness, no matter how "successful" the world thinks you to be.
* This post was sparked by a comment on PandaWhale
** This thought was sparked by a conversation with Kevin Miller from the Unreasonable Institute
*** The original insight behind this comes from Lois McMaster Bujold's excellent novel, Barrayar, though many others have explored the distinction between reputation and character.