There's nothing fun about being in love, Sammy was wrong. Following the dude round like a long lost puppy and slaying everything that could possibly hurt a hair on the angels head, just so at the end of it all Dean could turn to him and smile?
No. Love's a bitch, and Dean hated every second of it.
He would wakeup in the middle of the night, scared because he didn't have his angel. He had to fight the urge to kiss him, even though the feeling pulsed throughout his body. Telling himself he was fine, that it was just because the angel pulled his sorry ass from the pit, that it was just a stupid phase, that he needed him like a brother. The demons he had slayed because the said they would harm the angel, the monsters he faced having to keep his poker face steady as they mention the angel's name, biting down on his lip as the name rolled off the tip of their tongues.
But it wasn't just the evil, even his friends could pick up that something was up when the angel joined them, even his own brother could tell. But he kept the act up, denying it to his last breath even though he couldn't take lying to sam.
The worst part though? The way Cas would do the littlest of things that made Dean melt. The smile he gave Dean last christmas, the awkward conversations he really should've avoided but seemed to jump straight into, the personal space issue making him press up to Dean when he flashed in and out of room, the way he would tilt his head if anything mildly normal would be mentioned like cartoons or a sci-fi reference, the way he watched over Dean as he slept which in hindsight was stalkerish and creepy but also sweet, the way he was so innocent and didnt understand most things, the moments where he would smile at Dean when all seemed lost, the 'more profound' bond as he put it.
Cas would always be in Dean's mind, like a tumour latching itself on and not coming off. He would think about Cas at the most awkward of times, and I mean the most awkward.
Sam could sense Dean's discomfort, yet the only thing he could say to him? Love is fun.
The pain Dean went through for this one being trapped inside another man's body, it made him crazy. At times he could even boil over with jealousy, just by a comment some stranger would make about how handsome Castiel was. Sometimes it would be hatred, hating himself for feeling that way, hating cas for making him feel like that, hating the cupid that hugged him because he was pretty damn sure that he had something to do with this. Other times it would be self pity, crying because Castiel wasn't in his arms, pouting because Cas would choose Sam over him for the silliest of things, sulking because Meg had kissed Cas and he hadn't. The rest of the time he would crave Cas, craving his eyes, his hair, his hugs, his lips, all of him. It wasn't Jimmy he was falling for though, the angel's vessel had nothing to do with how Dean felt. It was Castiel who saved Dean, Cas who gave him the long stares, Cas who would be bewildered in the presence of a back to the future quote, it was Cas Dean loved.
So no, love wasn't fun. It was a bitch, it was cruel, it was unfair, it was selfish. If lust was still on earth she would pride herself on the sin purged into Dean Winchester. But it was also love, the reason Dean would get out of bed in the morning, the star that he followed, the thing that held him together, and as much as he hated love... love made him the happiest person in the universe.