I've found that the biggest factor that keeps me out of the gym is boredom. January thru March, without fail, you'll see the same new faces come into the gym and do the same 20 exercises in different variations. Then they disappear slowly. I've been there, just get bored out of your mind. When I made that realization that people were quitting on their goals because they they got bored, I realized I had done the same thing cyclically for years and years. Couple boredom with having kids, depressing, the season's, etc etc etc, and it's easy to find excuses not to lace up the shoes.
So, now, I add things in all the time. Or, I change up my routine when I feel like I'm in a funk. Spin classes, Cross fit, weight vest training, yoga, sand bags, kettle bells, distance running, sprints, powerlifting, hill training, mixing workouts into my range shooting, etc etc. I've done it all, suck at most of it, but keeps it fun.
That's just the method I've found that works for me.