Success breeds some weird shit, especially if you were close to people, which happens a lot in startups with a high work ethic. Friends turn into strangers, friends turn into enemies. Some make more money than you — a lot more — and some make less than you — a lot less. Brews some weird undercurrent sometimes. Money changes things. Even if you don't make real money... the perception of success changes things, too.
The road to a "successful startup" can be paved with a lot of bullshit. Burnout, depression, stress, mistakes. Regret. Employees locked in their handcuffs, even though they hate their work and their lives. Taxes. Paying people to advise you on all of these things. Trying to do something bigger the next time. Trying to move up and forward, bigger and better. Recapturing the lightning in a bottle. Dealing with yourself, and your reactions to all of this. Feeling uncomfortable about those feelings.