I became a Celtic fan because when I started following the NBA, the first game I saw (in 07-08), Rondo struck me as a ridiculous odd force of nature, even amongst the big three. So let's say we swap Pierce for Rondo in the question, because ultimately, that doesn't matter.
Over time, however, I fell in love with their defensive focus, and the way the Celtics organisation carried itself over it's grand history, spanning several dynasties, with a great many retired numbers and famous coaches.
Hard work, teamplay, defense first. Between Larry, Red and Bill, there is nothing any franchise can offer that will top the Boston legacy.
Hence, I'll bleed green 'till I bleed no more.