There really aren't any valid reasons. It's pure childishness. Putting emotion over reason is never helpful, based on ability there is no reason Rooney should leave this football club. Let's face the reality, we are not in a position in which we can sell our best players on a whim. People need to stop being arrogant. Manchester United are not the only elite football club in the world, there are several clubs on a similar level, and at least a few are more attractive to players. As fans we should be extremely thankful we have players the calibre of Rooney, instead of throwing a tantrum because the player may have entertained the notion of playing elsewhere. It's pure fantasy to expect every player at the club to worship it in the manner we do as fans. It's a really immature perspective. Wayne Rooney has achieved wonderful things for this football club, and is one of the best players in the world. So long as he is here, we should support him, and if ever he does actually decide to leave, the mature thing to do is to thank him for his service.
The important thing is to be realistic. At least four other clubs in the Premier League are now at least at our level. Nothing can be taken for granted, and with the uncertainty brought about by Moyes, Woodward, et al., attracting players of the required quality is not a given. Keeping world class players like Rooney, and potential world class players like Januzaj must be a top priority. This arrogant notion of 'no player is bigger than the club' needs to stop, or reality will soon hit these fans hard.