It annoys Dean to no end that people seem to think that he's just totally oblivious to the fact that Cas clearly has some sort of thing for him. How can he not know? Honestly, sometimes Cas acted like a preteen girl with her first crush.
Only someone who was blind would miss the glances thrown his way when it seems as if he's not paying attention, the way in which Cas relaxes whenever Dean's in the room, the looks of worry that rival Sam's when Dean gets hurt. Of course, the guy wasn't very subtle. He never answered when someone besides Dean tried to call him, and when Dean called, Cas was there faster than should have been possible.
It makes Dean furious, unreasonably so, when people start talking to him about how he has it bad for Cas. Not because he was trying to deny it, far from it. He just didn't like to think about it because he hated it because he hated the way in which it unsettled him.
Very few people made Dean feel such a way, as if he'd be lost without them there. Truth be told, he could only think of one person right off the bat that he had felt that sensation so powerfully toward. Even then, it was out of respect, not out of love. Dean had loved their dad, yes, but the amount of respect he felt toward their dad far outweighed the amount of love he felt, and rightfully so. John Winchester had demanded respect from both of his sons. Dean had learned rather quickly that giving the man the respect he wanted made his life so much simpler. It made it easy on everybody, especially him.
Cas unsettled him in a different fashion. Dean knew exactly what to call how he felt toward the angel, but he refused to let those words slip out of his mouth. That word was for family, and even then, it was rarely used. It was a weakness, in a sense. Using that simple word meant that somebody, something, out there could hurt him so badly that he might never recover from it. That word meant that someone, in their own special way, had some sort of power, some sort of control, a hold, over him.
Dean had gotten along just fine on his own for years now, and he would continue to do so. He didn't need anyone. Well, he liked to think that he didn't need anyone. Deep down he knew that without some people, he'd be lost, adrift with no anchor to keep him grounded.
Without Sam, he'd be lost. Without Bobby, he'd be lost.
Without Cas, he'd be beyond lost. Nothing in this world or any would be able to save him or stop him, especially if he let himself admit that he loved Cas.
That thought kept him at bay, kept him from doing what he really wanted. That thought kept him anchored to some form of sanity, but it also drove him crazy.
