Top reasons given so far seem to be:
1. Master of my own time and/or no more alarm clock
2. No commute
3. No office politics or crappy bosses
4. Ability to take naps
5. Freedom to travel whenever without worrying about vacation time.
BINGO!
Totally playing devil's advocate here:
1. Master of my own time and/or no more alarm clock--Bored! Even working I have plenty of free time.
2. No commute--commute is really easy now
3. No office politics or crappy bosses--office politics are bare min and bosses are cool.
4. Ability to take naps--go to bed earlier and you won't need a nap
5. Freedom to travel whenever without worrying about vacation time.--I go on plenty of trips now.
OK, Those 5 are all good things when retired. Those responses I added are some things I have heard from people when talking about ER.