It all started because you picked up a coat.

You noticed that a man had dropped his coat and decided to pick it up for him. He was very handsome and looked up at you with an amazed expression as he thanked you. Turns out, that man is a selkie and giving a selkie their coat officially means you are married. Now, he had run into you again and was handing you a ring box. "I thought we should get married the human way as well." He says when you ask why he is giving you a ring.