It's easy to think that Dean was selfless. Always taking care of his little brother. Some kind of hybrid between parent and sibling. And the dual instincts of protectiveness overlapping, amplified, coalesced into subconscious action. That's why he could still be a jerk to Sammy in one breath, and save his life in the next. There was no cognitive dissonance because there was no cognition. It wasn't as if he constantly sacrificed his own wants and needs for the sake of Sam; he had no needs outside of Sam. It wasn't a conscious decision. There was no "Should I eat or feed my brother?" it was more like "Better save that, Sammy'll be hungry." Consciousness implied that there was a decision, where there really was simple instinct. Dean didn't make Sammy his purpose. Sammy was Dean's purpose. Not because he chose for it to be that way, just because it was.
To think he wished differently would be misleading. Again, to wish implies choice. And there was never a choice.
So it wasn't exactly selflessness that led him to the crossroads. Desperation, sure. Pain, a given. Grief, mind-numbing, soul-crushing grief, certainly a factor. But it was more instinctive than that. Thoughtless almost. It was a foregone conclusion with a façade of premeditation. Do not pass Go, do not collect $200. Perhaps he had flipped the card, but that was the rules. It was selfish in the extreme, he knew that later, but what else could he have done? He could no more live without Sammy than a train could run without tracks. It would be aimless, messy and short. Very short.
He knew this was going to hurt Sam, knew it firsthand. Better than anyone maybe.
But he didn't care. At least, not until it was too late.
He was simply getting back on track, regardless of where it led.