The Lady Laughs

Or if anything happened to Sam. It's pretty much an unresolvable loop. Dean believes John values Sam more than him. In order to disprove that - that John loves Dean equally - there would have to be a situation where both Dean and Sam are in danger and John somehow picks Dean to save. But in that scenario, I can't see Dean forgiving John for that, or being able to live with the guilt. And if John picked Sam, even though Dean would on one level be relieved, on another level it would be just what Dean expects to happen. I honestly can't think of a scenario that would prove to Dean that John loves him just as much as he loves Sam (there could very well be one, I just can't think of it). The thoughts and beliefs are just too ingrained at this point.