Try some of the following on for size...
1. Work has begun to interfere with all the many other things you want to do - fitting them all into weekends and holidays is becoming more impossible or unpalatable.
2. Life just seems too short to carry on working.
3. You crave your freedom.
4. You want to have control over your time.
5. You want to retire while you have the health and strength to do everything that you enjoy.
6. You have a sense that you would regret continuing to work full time if you did it for much longer.
7. Your gifts and talents are not being fully utilized.
8. You're not even sure what your gifts and talents are!
9. You're prepared (and able) to lower your overheads if you need to.
10. The kids are finally off your hands, the mortgage is paid off and your debts are either paid off or well under control.
11. You would be able to cover your spending needs if the condition of your health changed.
12. You've successfully lived on your projected retirement income for the past six months.
13. Your partner is retiring and you don't want to 'lose track' of each other.
14. You know you could do your job perfectly well until you die, but you're no longer happy or fulfilled.
15. You plan to have a very active retirement.
16. You have plans for what you will do with all that extra time.
17. You have a plan for turning your hobby into a business.