Ignorance is bliss. Ignorance is bliss. Ignorance must be bliss.
Because if not, then the Winchesters live a lie.
What they don't know won't kill them, right? Because Sam can't know about the way Dean thinks of him, and Dean can't know about the dreams Sam has had. If they ignore - not deny, just ignore - the fantasies and the longings and the deepest darkest desires, then really, what's the harm?
But sometimes they wonder. Sometimes they wonder if bliss really would come in the other's knowing. If Dean felt the same...or if Sam had thought about it too...then years had been wasted dancing around a heavily guarded truth.
Years can go by in this way; and they have. Ignorance is bliss. What they don't know won't kill them. And truly, they have better things to worry about. Though maybe part of the reason Dean enlisted Sam's help (a selfish act like no other) to find their dad was indeed his hidden lust towards his brother. Perhaps Sam only continued with the demon blood because he deserved penance for his sinful thoughts. And all that time in hell was time Dean spent staying strong for Sam, and Sam slowly broke down in the cage not from the torture but from the crippling longing for his brother. Every battle they fought they fought for each other, but it wasn't enough. Time after time, their actions spoke louder than words but both Winchesters refused to even listen for fear of being heard.