Another Reason
If there really was tension between the two couples already, it certainly did not help the situation when the Queen chose Harry to be the Youth Ambassador to the Commonwealth. Some people say that William took it very personally and was hurt by the fact that he was not chosen for this position. Though he did have the power to veto this decision, he chose not to.
Another Reason
Confirmation
Many people who absolutely adore the Royal Family like to think that they are not actually facing conflicts, but rather it is just the media trying to start drama between them. One time, however, the four were all in an interview together when they got asked if they ever find themselves in disagreements with one another. William spoke first to break the silent pause and said, “Oh yes.”
Confirmation