Laying down on the backseats of the Impala wasn't exactly how Sam wanted to spend the journey to Bobby's after a hunt - especially if Castiel was going to pop in on them, quite literally. He had a tendency to appear on them, no matter what time of day, or what they were doing, without much warning. Sa didn't particularly want an angel sitting on him, when - well, if - he did appear.
Unfortunately, he didn't have much choice. He had damaged his foot pretty badly during their last hunt, and he had to keep his foot elevated, to decrease the swelling, and the only way to do that was to sit in the backseat.
It wasn't bad enough to warrant a stay in the town, and Bobby's was only about an hour's drive away, so they decided just to sit the younger Winchester in the back, and for the elder Winchester to drive slowly over particularly bumpy stretches of road.
When Cas appeared, he arrived into the front seat, slightly closer to Dean than was deemed appropriate, but the blond didn't seem to mind. Especially because it was Castiel - the angel that had no boundaries, when it came to invading people's personal space, but he seemed to know even less when it came to Sam's older brother.
Still, if Castiel made Dean happy, how wasn't going to complain. He might tease his brother with how he used to always complain when Sam would do something remotely feminine - "that's so gay, Sam" - especially when he was the one who turned out to have some homosexual tendencies. Well, bisexual. The younger man was always catching his brother staring at some woman's boobs, so there was no way that he wasn't into girls too.
—
About a month later, Sam walked into Bobby's kitchen, wanting to get himself a beer, and caught Dean and Castiel locked in a rather chaste kiss - though it seemed to go on forever. The younger man was able to quietly get himself the drink he wanted, and got out of the room before the pair pulled away.
Later, he would find out that was their first kiss, and Sam couldn't help but smile at the fact he was there to witness something so important - and normal, for once - to his brother.