One of my favorite things about training for strength is being way stronger than you look. Nothing is more satisfying than someone saying you can't lift something and then you do it with ease. It really taught me not to judge anything by its cover.
What keeps me going is that I am 51, and I feel better than I have for years. I have gotten stronger, a little fitter cardio wise, and I can see & feel the transformation from sticking with the program(s). Plus, I bought a 32 kg kettlebell last year, and I plan to own it by the end of next year, with consistent work.
I enjoy the challenge, learning about programming and if I'm going for a heavy-ish press squat GU or swings I have a clear mind. I don't often have that so training for me is time to foster an meditation practice
I will turn 47 in January, and never started exercising until 10 years ago or so. I compare how I feel now to before then, just general movement ease, endurance, flexibility in my life, lifting heavy things/odd objects without having to ask for help, and that is a motivator in itself. I have been working on S&S for a while now since June 2016, and just lifting and swinging feels good....it's like a moving meditation, and it gets the blood flowing and feels very satisfying
I also like challenging myself toward a goal (Simple) and building character in myself. It's fun to see a goal get closer.
Another thing that motivates me is what I see around me. The use it or lose it concept. I am an RN, and have seen lots of things like fractured hips, people who cannot get out of bed on their own or need some sort of help (even people like only 15 yrs or so older than myself) Or working now in Home health and seeing people who have all their mental faculties, but are in an assisted living facility by virtue of not being able to move well, squat to a toilet and stand back up, lift their leg over the edge of a tub. So sad. My grandma....a huge inspiration to me....was a Ukrainian immigrant to Canada/farming stock and lived to the age of 91, still gardening the day before she died. My goal is to still be doing things like that. Strongfirst principles play a huge part in being able to do so.
Habit. I am definitely a creature of habit. If I can build good ones, then I just have to stay the course. Consistency works its magic over time.
One thing I've noticed with my personal training students is that the morning ones are the most consistent. The afternoon ones often cancel; something comes up, not feeling well, etc. If you make exercise first in your day, there is a lot less that can knock it off the agenda.
Had an interesting conversation a little while ago with a coach talking about an ultramarathoner who just keeps running and training because she's trying to figure out where her actually limit is - she just hasn't found it yet. I think that's a probably the mark of real internal motivation - great athletes don't do it just to win, they're trying to figure out exactly how much they are capable of.