Only when you don't enjoy that particular from of exercise. If it is fun, that's the only motivation you need.

I used to struggle with anxiety and depression, until I found that 60-90 minute walks would clear my mind and help me feel much better. The effects wouldn't last very long, though, so I walked around five days per week, or around 30Km per week.

Then I bought a bike and found it much more fun, which means I look forward to my bike rides. Because of that it doesn't feel like a chore. It also turned out to be more time efficient, and the mental benefits of a single ride last for a few days, so if I catch a cold and can't ride ride for a few days I am not crawling up the walls like I did back when I only walked.