Well, i think you should find out, what makes humans happy.
As far as I've discovered:
- prove own knowledge, skill and ethics
- collect things (more varied the better)
- listen to stories
- be creative
- be social
- gain fame (especially, liked from people you know)
- change, variation (even a little worse change is good)
- be bad (tease your friends and foes is funny)
- do things in rhythm (music-games or precise routine-movement mostly in fps)
- have a advantage (be good in something or know something your enemy don't know, hiding games)
- look at arts
typical single-player boss-fights ends up in a prove of skill and some knowledge, collecting treasure, sometimes a continuation of a story and sometimes a new level (change, variation). you can do without it or replace it, to make a good game.