It happens to all of us. Some days, climbing (and especially training) just doesn’t sound fun.
The question is whether or not you should do it anyway:
Show up and do the work you said you’d do?
..or take the day off and recharge?
It depends, and it changes. But deep down you probably know which one you need today.
Whichever you choose, embrace it. No reason for guilt. You are doing what is best for you.
And don’t worry about the fun. It always comes back.