There is a difference between having fun and being happy. Often people will say, “You hiked the AT? That must have been so much fun!” And I laugh. Because it really wasn’t fun for me most of the time - I hurt too much, too often. There were fun moments, but not all day every day. Still, I was happy on the trail. I loved the life, the lifestyle, the people, the community along the trail, the natural beauty of the woods and the mountains, the wildlife, etc. So I would put up with the not fun parts (pain, rain, pushing too hard, etc.) in order to enjoy those things that made me deep down happy. It’s determination that will get you through the notfun parts when you know that the reward --happiness in my case-- is much greater than the cost.
There are those who seem to keep going purely out of stubbornness, they seem to get no pleasure out of their hike, but their reward may be different - pride in accomplishment perhaps - and for them too, the rewards are greater than the costs.
One way or another, you aren’t going to keep going if you aren’t getting something of value to you out of the experience. For some, it’s fun, for others happiness, for others pride - YMMV. As others have said, life is too short to waste doing something that gives you no joy.
Spirit Walker