The team spirit within the team will most of the times increase when you have another team to compete against or a common “enemy” outside the team.
This is something Mourinho is using many times, creating enemies outside the team to make the team stronger inside. I think Mourinho has already couple of scenarios ready, Barcelona is the favorite to win the La Liga in everybody’s eyes and the referees will give advantage to Barcelona… (this was written before the season started) Try to Google some of those sentences and see if you get any hits.
“I knew this would happen long ago, but I do not care. I know because I know who controls the game, and if you look at their statements, then you understand why they have made this decision. I worry about playing on a Saturday night at 22:00 then on a Tuesday in the Champions League. Barcelona [does] not want to play on the Saturday because they play on the Wednesday before, and because of this we need to play on a Monday.”
As a leader you need to take responsibility for the extreme we and them situations that might occur in sports. A common fight against an “enemy” outside the team will strengthen the team spirit, but might in the long run create an unhealthy hostility against other teams or groups and it should not be necessary to do this in youth teams. You should focus on the challenge or compete against the situation rather than an enemy outside the team.