This is why I left. The words echoed so starkly in John's mind that he has to stop and ask Sam to repeat himself, sure he heard it wrong, though he can't for the life of him figure out what Sam said if he did mishear.
But of course, he didn't mishear. Sam's anguished anger shone so clearly in his eyes despite the dim light, this low snarl in his usually gentle voice grating in his ears. If John had been less angry himself, he might have regretted being the type of person who could bring such emotions from his sweet little boy. At least for a little while, until his determination to end this and keep the boys safe kicked back in.
And then Dean was between them. Dean always tried so hard to keep him and Sammy from fighting. They were both so stubborn, sometimes there wasn't much he could do besides physically get between them, but he tried so hard. He'd always hated it when they fought, and John could see that it had only gotten worse since Sam had left. That, more than anything else is what made him let it go, that and the fact that Sam and Dean were both in the Impala waiting for him.
John could see it all so clearly though. Sam would leave again. Maybe not now, but as soon as they got this bastard. And his constant questioning would slow them down enough that they might never get to him. And really, how much was he protecting the kid by making him angry? Sam only got more reckless when he was angry, and neither of his boys were the most cautious sorts.
But dammit! There was a time and a place! He actually didn't have a problem with Sam's inquisitiveness, his baby boy had always been that way. But when he decided he needed to know something, he thought he needed to know it now no matter what, and that was something John just couldn't handle when he was in the middle of planning, or tracking, or, hell, just about anything besides preparing for an attack. Even then, he was more instructions-and-figure-it-out-yourself. But his bright boys figured out everything they really needed to know, and was it really his fault that he sometimes forgot that the boys were old enough and practiced enough to be able to handle just about anything he could throw at them?
It wouldn't hurt anything to tell them about the Colt, though. And maybe knowing he had the gun -because he was going to get the damn gun- would reassure them that he could face down that damn demon and come back to them. He'd seen the fear in their eyes every time he'd insisted on doing this by himself; telling them might be the only way to keep his determined boys out of the crossfire, since he knew they'd do anything to protect him, just like he would for them.
Yes, he decided, the Cold was a safe enough thing to tell them about; knowing he had it, they would be able to keep their heads in the game better when he left again.