Robby didn't get his happy ending in this story, nobody ever wants to lose the one they love, hopefully he will enjoy happiness again in his young life.
As for Darren, well really he did get his happy ending. I know that sounds ridiculous, but if you think about it, it's not as ridiculous as you first think. It's sad that Darren died at such a young age when he is yet to achieve his potential, but he died knowing that he was loved by the boy that he loved, pulling out of Robby's drive he was no doubt happy and had a smile on his face. So in a way, although it is sad, he did have his happy ending.