Try to take the sports fandom out of it, and inject more of the "business".
Say your boss comes in, and says you're fired, or you're being transferred against your will to a really undesirable location. You tell your family, and then your boss comes back and says "nevermind, we can't do that after all." Four months later, you get a job offer from a rival company, that promises you a better standard of living than you had previously, and which really seems to embrace and respect you and what you can do for the organization.
How many of us say, "Even though my current employer tried to terminate me, I'm still loyal, and would never join a rival company"?
I wouldn't, and I suspect the vast majority of Celtics fans -- or people in general -- wouldn't, either.