Dean was a hardened hunter.
Since he was a child he'd killed an innumerable number of beings. Sure, the majority were monsters but nevertheless, most had been in human form, had acted like humans, talked like humans, and at times begged for their lives like humans.
It was impossible for such massacre and death not to prey on a man's soul and Dean, although his hunting skills were bordering on the supernatural themselves, was still eminently human.
That's why when he looked at his little brother, the part of him which was pure and untouched, rejoiced. Sam was what kept him from turning into one of the monsters he hunted. It was Sam who kept him sane. And Dean loved him utterly and completely.
When it came to his baby brother there were no heights he wouldn't reach nor depths he wouldn't fall to.
That's what made Dean Winchester so dangerous. His love for Sam was a two-edged sword. It could cause Dean to act like an enchanting five-year-old, making Sam grin and shake his head in fond disbelief, but it could also turn him into a dangerous predator when said little brother was in danger.
And no sane person, monster, angel, or demon would be willing to take on a bedeviled Dean unless they knowingly wanted to step through the door to oblivion!