I guess I have some motivators at both the "strategic" and "tactical" levels. Big picture...I genuinely enjoy riding, so that might be my biggest motivator. It's almost borderline addictive, but it beats other things I was doing...so I carry on. Secondary to that is my desire to be competitive in XC mtb and gravel racing...that at least motivates me to do the workouts I don't want to do, like indoor sessions and strength training. This one is a distant third motivator and kind of a dumb reason, but I feel accountable to ride just by friends/family/Strava followers.
As far as continuing to enjoy riding, I do try to "hack" my dopamine to a certain extent. If I stick solely to riding outdoors, I constantly have to keep "upping" the rides to "maintain" having fun at them. So I incorporate about 2-4 boring zone 2 trainer rides in a week...I don't enjoy them, but I feel as if it "resets" my dopamine base. It allows me to keep enjoying my outdoor rides without adding unsustainable amounts of additional volume, intensity, new miles, etc.
At the "tactical" or small level, I do employ some "tricks" for when I don't want to do something. I don't particularly enjoy the indoor sessions, but I'm able to remind myself that it's only an hour or two, and that it will be over soon enough...basically that an hour is a pretty small amount of time, even compared to just the rest of the week. This is even a concept that I apply to hard climbs on outdoor rides or tough races. My other motivator is that I imagine the future success or fitness I'll have if I can just get through this "meh" workout. My final on-bike motivator is just self-coercion...basically, I employ my fear of looking weak or soft, in order to finish a tough ride or event. It's not my main motivator, but is a good last-ditch tactic for when all else fails.