The Endowment Effect is an important thing to understand when you’re a die-hard Celtics fan.
In psychology, the Endowment Effect has demonstrated that people will usually value an item that they already have higher than an item they do not, totally independent of objective value. Give some people a pen and others a mug and ask if they want to trade; the pen group generally kept their pens, while the mug group kept their mugs. Which was more valuable? Apparently, it didn’t matter.
In sports fandom, the same can be true. We are situated so close to our beloved teams that it can become impossible to objectively analyze the value of players relative to their cost.