lokitapendragon:

My headcanon is that Deanna Campbell, before her last name was Campbell, had a relationship with a mysterious drifter-type. They were clearly in love, but he was called away back to Wales, and she never told him she was pregnant. She met Samuel Campbell a couple months later and they were married before she gave birth to Mary.

That drifter was Captain Jack Harkness, which totally explains Dean’s womanizing con-man ways and the brothers’ inability to simply stay dead.