None of those options, having picked up my dusty guitar again 2 months ago, and practicing daily now, I can give you my reasons. It's fun to see my progress, I'm being creative with it which is also fun, and last but perhaps vital for the moment, I'm taking private lessons every week so there is some sense of responsibility, and the will to impress (instead of disappoint by obviously having done no practice). I think practice buddies could do the same if dedicated to.

But ultimately there's no 1 solution, and it has to come from the person themselves to want to keep going, whatever helps them do it. I can drop from my lessons whenever so I can relax more and not practice silly songs, but want to push myself.