They're unselfish - both in the world and on the court. They've built most of their team through the draft and player development - they're not chasing superstars. They're in a small market (by the NBA's definition). And you may not get this having followed them from a distance, but they're a family - they all really love and look out for each other. Also, Jokic is sincerely a great guy. Great teammate. Great husband. Great father. Great brother. Great citizen. Maybe my favorite player ever, in any sport - and if you knew me, you would know how high praise that is.
Even as a lifelong Nuggets fan, I can't think of many reasons they're not the most likeable NBA team in at least recent history.