Why has your favorite game stayed with you?
Well if we are talking about Dungeons & Dragons then I think it is because of all the endless possibilities. If I only ever played D&D for the rest of my life I don't think I would run out of ideas. There is always something more to be done, some new dungeon, hex, or plane to discover and explore.
Maybe that is why it has also been the king of RPGs for nearly 50 years.