Prince Harry and Meghan Markle could have done more in the royal family but they were forced to leave because their status as the "royal rock stars" started to overshadow Prince William and Kate Middleton.

William was destined to sit on the throne after his father, Prince Charles. Harry also knew since they were teenagers, that he would always be the spare to his brother, the heir.

But, Harry was apparently not always comfortable with this role, according to the new book, Battle of Brothers, from royal biographer Robert Lacey. From being the loyal third wheel to William and Kate, the Duke of Sussex apparently started to resent the institutional system more when his brother extended his family.

Things were even clearer for Harry when he met and married Meghan. Together, the two forged a partnership that became one of the most talked-about in the world.

With his wife by his side, Harry apparently saw his own potential and found a new purpose in life. Lacey suggested in his book that the moment the world also took notice of the Sussex pair, then, William and Kate apparently didn't feel happy.

Speaking with Good Morning America, Lacey said that the system that made the British royal family what it is today "can be cruel to the spare." Lacey said that spares of the previous generations, such as Prince Andrew for Prince Charles, and Princess Margaret for Queen Elizabeth, also went through similar problems.

The spares start off as the number two or the co-star of the heir until the heir gets married and has kids, pushing the line of succession down. This apparently made Harry unhappy about his future in the royal family but there was Meghan who gave him this window to "break free" and shine under his own spotlight.

Lacey also said that Buckingham Palace didn't know how to handle Harry and Meghan's popularity. The senior royal staff members were extremely challenged in dealing with the Duchess of Sussex because she was not willing to adapt to tradition and long-standing customs.

Harry and Meghan also ruffled feathers when their popularity started affecting Prince Charles and Queen Elizabeth's popularity. Lacey claimed that Palace aides then started to "edge out" the Sussexes to suppress their rising fame and this was evident in the Queen's last Christmas speech that deliberately didn't mention the Sussexes.

Lacey advised the Sussexes to "remember their place" as the backup and to stop trying to be more prominent. He said that, in the end, the most important thing is their relationship with the family.