Sam knew it was the turning point, that if anyone found out about this there would be no hiding their relationship for what it was; that Castiel could not pretend that this was entirely normal, even for humans. He knew it was more than he'd ever expected to get into, that it was one hell of a responsibility.
He also knew from Castiel's expression on seeing the gift that it was something Castiel needed, and something he'd started to need himself. Castiel trusted him, and he wanted someone whose trust he wouldn't destroy.
Sam fixed the collar around Castiel's neck, looped a finger through the hoop and pulled him in for a kiss.
"It looks good on you," Sam said, before attaching the leash to the hoop and wrapping it around the bedpost, loose for now. "Stay."
Nothing was keeping Castiel from leaving, but Sam knew he would obey nonetheless. The important thing was that this way? He had the choice not to.
It had started when Dean kept pushing Castiel aside, dealing with his own problems, stopping only to give Castiel the occasional task here or there or to chew both of them out for not doing enough to help him. Following Dean's attempts to fix the shadow Sam's apocalypse had cast was fucking depressing, but Sam followed nonetheless, knew it was his responsibility as much as Dean's to try and fix things.
Castiel followed because they were the only people who weren't trying to kill him and because he had nowhere else to go, and that was worse; Sam wasn't completely familiar with how Heaven truly was, or how its agents were supposed to behave, but he knew enough to know that being left to tag along without real purpose was killing Castiel slowly in its own way.
After God bailed, turned up in words only to tell them to leave Him alone, they'd all been shaken - but Castiel worst of all. Sam had pocketed the amulet and waited and waited for Castiel to come back to them, wanted to talk to Castiel, to give him some hope, even if they had none.
Castiel had pushed them away even as he followed them, and Sam had been left trying to hold Castiel and Dean together and find something, anything to grab onto, to call itself a reason to carry on.
When Castiel came back to them with a scarred chest and a newfound tendency to sleep, Sam had taken him aside and asked what kept him with them. Castiel couldn't answer, and Sam had wanted to ask more, but something had hit him - something about Castiel's attitude, about the way he wanted to answer but couldn't quite.
He told Castiel if he wasn't going to answer now then he wasn't going to answer ever, before telling the angel to go to bed.
Castiel had protested that he didn't need to sleep, and Sam had reminded him that yes, he did now, and if he knew what was good for him, he would.
Castiel had looked at Sam strangely before going to bed, and Sam had been shaken for a moment, but brushed it off.
When Castiel continued looking at him strangely the next day, Sam had told him he shouldn't stare so much before making breakfast and telling Castiel to eat. Chances were Castiel still didn't need to do that, but it was better safe than sorry; he wasn't taking the risk of Castiel not recognising hunger and fainting on them.
Castiel had nodded, started eating, and Sam felt his mouth go dry.
When he went to the bathroom before they left, he jerked off, thinking of the kinkier times with Ruby, promised himself this was just something weird and brief and it was better to be safe than sorry. Dean had laughed when he and Castiel didn't look at each other for the journey, and Sam had been too preoccupied with the simple, surprising pleasure of hearing Dean laugh to comment on the fact Castiel really had cut back on the staring, if only for now.
When Castiel came to him with an armful of drinks for the ride and asked which they should bring, Sam knew the question wasn't meant just as a genuine query as to what people preferred. He'd answered, taken the drinks from Castiel, paid for them and went to load up the car, praying Dean wouldn't be around for a while yet.
His prayers were almost answered; Dean was in the car when he and Castiel got there, but told them to look after it while he took a leak.
Sam had turned to Castiel, asked, "Do you want me to kiss you?" and felt like a complete idiot.
Castiel's breath seemed to catch before he swallowed and replied, "Yes."
Sam swallowed too, loaded up the car. "I'll send Dean for beers when we have our next break," he said, feeling sweat rolling down the back of his neck that wasn't just from the sun overhead. "And you'll come to our room. Alright?"
Castiel nodded, and Sam wished he had time to head the bathroom and jerk off again, had to spend the entire car journey carefully thinking about anything but Castiel, the motel, sending Dean for beers, or anything to do with their immediate future.
When he got to the motel he found Castiel was pretty fond of "Yes", although he knew "No" as well, which was more of a relief than Sam would ever have expected it to be.
Maybe he'd felt a little guilty about accidentally seducing the angel he'd always thought of as Dean's, but Castiel also answered questions that didn't need "Yes" or "No" responses, and Sam had quickly discovered that Castiel knew as well as any human how affection took different forms.
Castiel loved Dean, without any doubt, but he loved him as Sam loved him; he'd die for him - again - if he had to, wanted the best for him, wanted him to be safe almost more than he wanted him to be happy.
Sam had also been kind of amused to find out that Castiel didn't actually like Bobby, that Bobby gave him a headache in a very literal sense.
He found out Castiel liked kissing more than being held, licked more than being tickled, rimmed more than being fingered.
He found out Castiel's kinks involved marking and making a mess, but also that nothing was more to Castiel's tastes than being told what to do, even when it meant fighting his instincts. Ordering Castiel to stop moving when the angel was in the middle of riding him to orgasm was as much a torture to Sam as it was to Castiel but it was worth it to see him break, see him stop moving and dig his hands into the sheets to help resist touching himself, and finally to say "Please, let me finish."
Sam had said yes, because he wasn't that much of a bastard, and Castiel had realised then that he could ask for things in return.
He didn't always get the answer he wanted, but he could ask.
They kept it relatively subtle around Dean most of the time, though Dean did tend to bitch that Castiel listened to Sam far more often than he listened to Dean; Castiel had looked at Sam for support and when Sam nodded, commented that if Dean listened to him once in a while then maybe he'd return the favour.
Dean had shut up after ranting for the next ten minutes on the road, just because he could. Sam had grinned, looked in the rear-view mirror at Castiel's expression, the stern concentration replaced by something visibly more relaxed.
"You should catch some sleep while you can," Sam said, lazing back in the passenger seat. "Don't think we'll catch much sleep when we pull up next."
Dean gave Sam an odd look in response. "Not planning on sleeping, Sammy?"
"Just got a bad feeling about any motel you choose," Sam had bitched in return, but he left it hanging, giving Dean a moment to work things out.
Dean gawped at first, keeping his eyes on the road despite clearly being distracted, occasionally seeming to think of something to say before apparently giving up.
Sam hoped he hadn't misjudged, but Dean finally smirked before saying, "Think I might let you have the room to yourself, Sam. See if I can catch a weird rash while I've still got the chance."
Sam grimaced at the choice of words but hey, he'd scarred Dean's brain with the idea of him and Castiel, maybe it was only fair to let Dean return the favour.
Besides, if it meant getting Castiel to sleep peacefully in the back of the car? It was worth the discomfort.