First you have to buy the friend, lover and family pack on the Road to Rule. Make the person love you by interacting with them. At first you will only think of you as a friend but once you get up to a certain level they will give you a quest to either deliver or bring back something. Interacting with them further (after their relationship quest is done) and you will get them to love you by taking them on a date to their place of choosing. Grab their hand, lead them to the place and kiss them. NOTE: If you are going into a dangerous area they can be killed. After they will love you and if you have a ring you can propose marriage.