1. The fans.
Rooney lost a lot of goodwill from the fans when he tried to engineer a transfer to Manchester City in 2010, and many more have turned against him after Sir Alex Ferguson alleged that he was seeking a transfer at the back end of last season.
2. No longer top dog
Rooney is no longer top dog at United. He enjoyed an excellent relationship with Cristiano Ronaldo between between 2004 and 2009, and was happy to share the plaudits, safe in the knowledge that he was still scoring a lot of goals (Rooney bagged 97 goals for Manchester United in that period, while Ronaldo scored 112).
The situation has changed significantly with the arrival of Robin van Persie, however, who bagged 30 goals last season against Rooney's 16. Rooney feels that he is now playing second fiddle.
3. Confidence and the need to be loved
Rooney is a confidence player, and needs to feel appreciated and loved. Being booed by the fans when he was on the team bus during the end-of-season parade around Manchester will have hurt.
4. Doesn't want to just be a squad player
With the World Cup coming up next year, Rooney is desperate to go to Brazil on a high and make up for his disappointing performances in recent international tournaments. Not being a regular starter for Manchester United won't help him.
5. A new start
Rooney probably feels that, after almost 10 years, it's time for a fresh start. He is not the player he was, and will no doubt hope that a new challenge will inspire him to new heights.
6. Relationship with David Moyes
He has history with new manager David Moyes, who once sued Rooney for comments made in the player's 2006 autobiography Wayne Rooney: My Story So Far.
Moyes also doesn't suffer fools gladly, and has coped well with selling Rooney before. Rooney will know that he doesn't have much negotiating power or stock as far as Moyes is concerned.
7. Burned his bridges
You only get so many last chances, and Rooney probably realises that there's no way back for him now at Manchester United.
Although the club still insists Rooney is going nowhere, it's difficult to hold on to a player who is determined to leave. The fans, too, won't be in uproar - many already seem to wish he would just go. And financially, it's a good time to sell him - Rooney is still only 27, and will command a big transfer fee which can be reinvested in new players.
image: © Audrey Pilato