Rooting for a particular team is just one of the many ways in which the ego strives to create an identity or feel worthy. By associating with a team, it can pretend that it is somehow connected to the players and then ride their coattails to "victory" so that it can feel as though it has accomplished something.
The ego needs to fabricate a reason to identify with a particular team. It’s usually something flimsy such as happening to live or have lived in or near the geographical area that the team "represents". For example, someone from Maine might become a Red Sox fan because that team is associated with Boston, which is the closest city to Maine that any baseball team "represents". If there happened to be another team that "represented" Augusta Maine, then folks from Maine would root for that team and want the Red Sox to lose.
The true self enjoys simply watching a game. It has no preference for who wins because it does not need to derive an identity or a sense of self-worth from anything external.
The ego cannot simply enjoy a game because there would be no possibility of obtaining glory for itself.
With the possibility of "winning" comes the possibility of "losing". When the ego’s team loses, it experiences negativity. It will get angry or depressed because the team that it identifies with has lost, which triggers its feelings of worthlessness.