I like that Hattrick is a slow game. Here, I have time to think everything over. I also appreciate that Hattrick gives everybody the chance to be the best because you can't buy in-game advantages.
One reason I play Hattrick is because I like the cyclical nature of the game. You start with an idea about what kind of team you want to build, then you train long-term (up to 3 years in real life!). While you build, you can test and try to comprehend the aspect of the match simulation that you want to master. I also like the tactical choices you have to make for each match. But the most important component is the community.
I love Hattrick because it's simple. It is not taking much time to learn the dynamics, but you can tweak it forever. I love Hattrick because to me it is like Subbuteo or Risk. It is simple to learn but hard to master. It's still exciting to wait for 90 minutes to see the result of a game, four months to see the end of a season. I love Hattrick because it is possible to set goals for several years, to see your team growing up, to learn the names of the players, and to build memories about them.
I would say the community is the strongest reason I have stayed in Hattrick for so long. The game too of course. After having trained my players and played in the French Division 2 for a while, I started focusing on training international players to help the Moroccan team. Today 80% of the defense of the Atlas Lions was trained by me!
Some of my Hattrick friends I've known for more than a decade now, which is longer than quite a few of my real-world friendships. You build up trusting relationships with them, and meet people you'd normally never meet. That's also what I really enjoy about the federations in Hattrick - some are really places where you can share your personal stories. A federation in Hattrick was one of the first places where I disclosed I was going to become a father!
What I like about the Hattrick community is that there are real people and you don't just compete against the computers. You play against each other, but you can also help each other in the forum and talk about everything.