Many times I chalk it up to the fact that my wife didn't want sex after menopause. That is part of it, but not the whole story.
Up until the time I got married, I had sexual relations with exactly 20 girls (including my wife). Now, statistically, that's not bad, but it was not great either (to me at least). There were a lot of times I said or did the wrong thing. Lots of missed opportunities due to shyness or indecision. Lots of flat out rejection. That frustrated and haunted me to some extent, especially as I brooded about it for the 30+ years I've been married. So in my case, I think part of it is a need to right those wrongs. Its a need to become the stud I never was, but maybe could have been, in my prime formative years. I'm in my late fifties, so this is my last shot. I don't want to be regretting what could have been on my deathbed. Obviously, I'm not the only one who thinks like that.