One of the major factors of a having a unified team is chemistry. It is very difficult to win if a majority of players are not working together. Each person has a specific role, and together you work to win games. A good example of this is the "dream teams" emerging in the NBA. The Celtics have been able to come together as a group, because their stars were aging already and accepted the fact that they would no longer be the superstars. However, a team like Miami is struggling to work out together because their stars are still in their prime and want the ball the majority of the time. Dream teams only work if the players sacrifice their statistics for the betterment of the team.

Pierce details his fears of ruining chemistry in the article: http://espn.go.com/blog/boston/celtics/post/_/id/4690965/still-together-pierce-likes-cs-chemistry