29M here. I was very active the first, oh, 23 years or so of my life. I was always skinny though. Great cardio/endurance. 6'1" and I always hovered around 150lbs. I've never had measurable muscle. Any exercise I got was almost always playing sports, therefore enjoyable. After I quit playing sports, I tried to force myself to run on a treadmill but absolutely hated it. Running outside wasn't much better. Forced and planned exercise was like torture, so I went 5 years without regular exercise.
A few months ago the doctor told me my testosterone levels were dropping and he recommended lifting weights and a change in diet to maintain or raise levels. Enter /r/gainit. I had no idea where to start but did a lot of reading online. I settled not on something that is widely recommended but something that works for me as a father and husband with a full time job and 45 minute commute. I use dumbbells at home. I use Beach Body routines. I do brosplits. I don't take rest days (I've had 2 rest days since I started about 3 months ago). 5 days lifting, 1 day cardio. A lot of this stuff is recommended against here, but it works for me. It took a few weeks of forcing myself to do it before it became second nature and I started to look forward to it. I don't even have to think twice or plan for it now. It's part of my daily routine. I just do it, and I look forward to it every day. It's part of me.
I think one of the biggest motivators for me is visual results. I haven't been doing this long, but in a short period of time, I started noticing results. Something I never saw my entire life while hovering around 150lbs. Tracking measurements and taking pictures helps a lot. I look forward to getting on the scale every morning and watching the number go up. I count down the days to the beginning of each month to when I let myself take new progress pictures. All of this and I've only put on about 8lbs of body weight, and I seriously think that 6lbs+ of that is pure muscle. It's an addiction.
...to add, my diet is just as much of a focus, if not more, as my workouts, but I didn't go into it since you're asking specifically about workout.
TL;DR: I found what worked for me and after a few weeks it became something I looked forward to instead of dreaded. I consider myself addicted now and would much rather workout than not.