Everyone is born with a tattoo on their dominant forearm. Their soulmate's first words to them are branded onto their arm in ink. Everyone has a clock, that they rarely get to see, that tell them how much time is left until they meet their soulmate. Castiel's ink is just one word on his left forearm; "Thanks." Because of how common it is, Castiel's parents have always monitored him. He isn't allowed to date, he isn't allowed to meet anyone without his parents' permission, and he's under constant surveillance in order to prevent him falling in love with a random person. Dean's ink is two words on his right forearm; "I'm sorry." He just made sure to not hang out with people who never fully said 'I'm sorry'; he just wants to make sure he doesn't accidentally fall in love with the wrong person again. Since his parents didn't really care, he got to make his own decisions.