He wasn't sure when it had happened: from the moment he met the prince, he had always been "Hamlet's Horatio". It hadn't made it easy at Wittenburg, but he really didn't care what the other students thought. If they wanted to tease him about the prince, let them. It wouldn't make him stop spending every waking moment with him, or talking with him about everything, or being found in the weirdest places with him. It wasn't awkward in context to be found in a tree together, or in a fountain, or dancing, it was just when people walked in at the worst possible moment. They had been discussing birds and then decided to climb the tree. Hamlet had been showing off his balance, then fell into the fountain. When Horatio laughed at him, the prince pulled him in too and they spent a quarter hour splashing each other. And the dancing... well... that was Hamlet's fault. He had been moaning at his inability to impress Ophelia with his dancing, so Horatio had offered to help. Of course, no one was interested in the back story when they found them. They just giggled and gossiped and were generally annoying. So Horatio didn't pay attention to them. He didn't care when the letters from Hamlet to him were intercepted: he had given up trying to explain that whole "He that thou knowest thine" business. People were small-minded and wouldn't listen.
Throughout all these instances, Hamlet had a sort of flippant fondness. He'd grin at Horatio and say and do all of the above things, but he'd do them in such a way that you didn't realize it was strange or awkward until it was over.
But for some reason, when Hamlet stilled his usual flurry of movement and turned all his attention to Horatio, carefully fixing his friend's bow tie, and told him he wore him in his hearts core, "Aye, in my heart of hearts", it felt different.
It felt like a farewell.