There were orders from Heaven to get John and Mary together in order for Sam and Dean to be born. At first, John and Mary could not stand each other, but soon a cupid came in and worked his magic. Even with the cupid's powers, the love felt between John and Mary was very real. Mary didn't tell John about her hunting background. To her, John was normal and could give her that normal life she craved. She ignored her father's misgivings about John, telling Dean, her future son and who was traveling to 1973 from the future, that she would run away if she had to, but that she was in love with John and that even though he had been through the Vietnam War, he still believed in happily ever after and was kind.