I have always enjoyed my daily exercise and my satisfaction from working out was my motivation. The older I get the harder it is to get my injuries to heal. Being in pain has started to take away some of my motivation. I gave up on pickleball because of an injury and I miss the social part of meeting up with others and being active with them. Also, this whole virus mess has cut into a 2-week biking trip and it is getting close to my biking and hiking season. So, most of my lack of motivation is from factors I cannot control. I will just keep doing whatever I can and enjoy it as much as I can.
Honestly, and however shallow this may seem, my motivation comes from gym-rat pals / colleagues of mine who know that I do not go to any gym and accuse me of having various heavy fitness equipment at home while all I have is the Hook + Iso-Strap. This tells me that what I'm doing is effective and I should keep doing it.
I wash my hands, stay away from people, and not watch the news (other than the weather). That motivates me to believe I'll enjoy another day and will want the capability to enjoy it or be ready for it.
Perhaps I misunderstood the question, well the main motivation is the obvious: feeling better i.e. Mobility. This is what fitness should be all about, since mobility = strength + flexibility. Body aesthetics (muscle mass & tone) are a consequence.