You ran into him at a shop one day and noticed he had dropped his coat. Being the friendly person you are, you handed it to him. However, Cassen is a selkie. According to selkie law, giving a selkie their coat is the way that they marry, so when you handed him his coat, you were officially marrying him. Now he stands in front of you holding an engagement ring in a box and offering to do the human customs for marriage as well.