In the Game of Thrones political power and military might seems to be mainly determined on a the house system (Lannister, Stark etc).
Under what circumstances is a house created? Marriages seem to amalgamate households but not create new ones.
Do people simply lose interest in their families and start their own? If so, where would they live - it was be unwise to give a new house a castle as it would create another potential enemy in the future and lose you a castle.