I think you guys that see fans not wanting to watch the Celtics when they are bad are confusing Celtic fans and bandwagon fans.
Fandom is like a hurricane category rating system
Cat 1 - Non Fan. Could care less if basketball ever existed
Cat 2 - Bandwagon Fan. Likes basketball, but likes being associated with a winning basketball team, hopefully championship winning team, even more so only roots for winning teams and tends to jump to another winning team when the original winning team starts losing or is no longer champion.
Cat 3 - Casual Fan. Likes basketball and is infatuated with one team but doesn't put much stock in whether that team wins or loses. Sure they are happy when their team wins and disappointed when they lose but it really isn't that important in their lives.
Cat 4 - Fan. Loves basketball and loves a team. Winning is joyful and losing hurts, a lot. Sometimes this means during the tough times they may not follow as closely but they are still loyal to their team.
Cat 5 - Die Hard Fan. Loves basketball and is devoted to their team through the best and worst of times and will watch every game including exhibition and summer league and watch the draft to see who their team drafts.