So what is it then?
The feeling of accomplishment as their farms are looking nice? Yeah, it's probablly the feeling of success being provoked, which is a positive emotion and feels good.
I think there is an element of escapism. I also think that escapism is the biggest factor in selling these kinds of games. There is something about the game that allows us to feel better about ourselves, more in control, more powerful etc. They harness the imagination and as we play our lives suck a little less. Not that our lives are all that 'sucky' only that it can always be better. Being Ceasar and leading your legions to victory is much better than going to work Monday morning.
I believe the best selling games are the ones that harness this the best. I believe that the more we are able to allow our players to escape into a world where they are much more than they can be in real life is the key. This is my basis on posts that deal with preserving the simulation factor of the game, keeping it as real as possible so that we are taken out of our illusion as we play as little as possible.
I don't think this is the only factor either, but I think it has the most bearing on participation. I also think that, in broad terms, factors differ in degrees depending on things like gender and age.