It's a pretty important question, given how much of an advantage it is.. As best I can tell it comes from any combination of these factors:

• intrinsic enjoyment of the activity
  • ideal, but usually not sufficient since there will be aspects that you dislike
• external pressures
  • family, immediate social groups in teammates or friends, broder societal expectations (imagined or otherwise)
  • probably the most common reason that anything gets done: institutional deadlines and/or social pressure
• hyperfixative/addictive tendency
  • of either the person, the activity, or both
• strength of one's sense of purpose/belief in what they are doing
  • being driven religiously or by some ideological mission applies well here
• mindset
  • optismistic, or defeatist looking for every opportunity as confirmation that they couldn't do it?
    • self-doubt

James Scholz, who came to prominence during the COVID lockdowns for his daily 12-hour streams of studying, seems to have been driven by a ideological/spiritual conviction. I've heard internet creators mention how an audience can push them to be more consistent and imagine that helped here as well.

David Goggins came to prominence through his motivational speaking about weathering extreme physical challenges like the Navy SEAL BUD/S selection program and ultramarathons in order to essentially mentally torture himself. It doesn't take a brilliant psychologist to recognize from his autobiography that his abusive father has cast a long mental shadow over him and that he is trying to prove himself over and over again in some mixture of internal and imagined external pressure. A rather grim source of motivation, but evidently effective..

I wrote about Tyler1 previously--he is well known for hellishly grinding competitive games in an iconic "brute-force" style. He's said he becomes addicted to them.