You need new goals, challenges, things to strive for to keep you engaged and make your life exciting and meaningful.
Otherwise you get into a nasty rut.

The whole concept of retirement as a time when you stop striving for anything is wrong.
People always need goals to strive for no matter how old they get.
Humans are not happy unless we are always working towards goals of some kind, things that challenge us and light a fire of passion.

If you stop striving, then basically you're not living.