The case that a princess who is already married to a king becomes pregnant , and their child inherits the crown of both countries is a rather common cause . But a personal union sometimes was seen as a means against uprisings if a state wants an annexation of an other state . Personal unions can arise for very different reasons , ranging from coincidence ( a princess who is already married to a king becomes queen regnant , and their child inherits the crown of both countries ) to virtual annexation ( where a personal union sometimes was seen as a means of preventing uprisings ) .