After Dean dies on a hunt way before he was supposed to, a reaper accidentally binds his soul to a stranger’s soul who died at the exact same time. This meant that any injuries or feelings - physical and emotional - that Y/n or Dean felt would be mirrored in the other person. After both of their souls are returned to their bodies, Sam and Dean track down Y/n and are left stuck together in the bunker whilst they figure out a solution. What starts off as anger and frustration in the beginning every time either gets both of them hurt gradually changes into tiny twinges of guilt over time, and it becomes impossible for them to hide their growing feelings for each other. As Sam gets closer to figuring out how to un-bind their souls, Y/n and Dean become suddenly aware of something: they no longer want to sever their connection to each other.