The Joker had an obsession with Batman and his family, to the point they believed they were cursed. This was not normal behaviour, that the Joker had a unhealthy fascination of Batman, and a love of torment for the Robins.
Each Robin attacked in different ways. Gradually getting worse, as boredom comes with each success.
The first Robin was kidnapped to a abandoned location and tied to a chair. Laughing the fist time they meet not knowing the joker had heard and wanted to make it like his own. So that's what happened, along with consequences.
Bit by agonising bit. Robin was given small doses of Joker's newest and quickly favourite toy. His very own special concision. A similar, demented version of laughing gas. The gas makes the victim laugh uncontrollably, their chests in pain, all leading to death with a broad smile on there faces, to match Joker very own crazed grin.
The small boy, no older than 9, had his laughter change from a pleasant sound that made others want to join in, to a creepy cackle, that sounded nothing like how it was before. Once satisfied the Joker thanked him for his cooperation and silently left.
Batman found Robin and took him home. It wasn't until several days later that the consequences had shown themselves. Dick Grayson, the boy was called, was simply relaxing, watching a comedy and giggled that quickly became something more. Dick taken by surprise clamped a hand over his mouth, and froze. He leapt over the sofa to update his father of what happened. Later on they found traces of Joker gas still in his system. Dick left out that laughing was now a stress reliever and that each time he laughed it felt like a weight had been lifted. The cure was in it's early days at the time, painful fits of non stop smiles and laughter were there to stay. Dick from then forth needed to take daily medication otherwise the painful laughter would not stop and leave him breathless.
But compared to the others, Dick got taken off lightly.
Jason Todd, the second Robin, due to a new discovery and betrayal, led him to Ethiopia and an timely demise. His new found mother had betrayed him to the Joker. Shelia Haywood watched with disinterest and a cigarette between her lips, as the Joker enjoyed himself, beating Robin to a bloody pulp with a rusty crowbar. Only to turn on her as well and ran with his goons, as a timer was set, and Batman arrived too late to save his second child from being blown up.
Begrudgingly and manipulated by, the third Robin came to be. Not wanted but sorely needed. After all Bruce had not taken to grief well, and needed help. Tim Drake, the third Robin believed that Batman needed a Robin to steer the darkness away. He may have helped Bruce but he too succumbed to Robins fate. Not long after Jason's resurrection and destructive events. Tim was taken and not found for a month. Tortured and brainwashed By Harley and Joker.
Joker Junior was born.
Weeks after being rescued Tim felt he didn't deserve to be Robin no more and retired from the position. Only helping out in the batcave when he felt up to it. Tim then partially forced Robin onto his new friend Stephanie Brown. Who accepted the position with unsureness, but hidden glee. Steph already had a go, by stealing old costumes and going after her father. She quickly fell in love.
But barely five months in, Robin was taken from under fer feet. Tortured too, but by Black Mask instead. When word got out Joker was not pleased and took matters into his own hands. It was a wonder how she managed to survive. Despite what happened, and her identity becoming public, Steph recovered, and decided to stay and help any way she could. It wasn't until years later after the death of Batman, that another Robin came to be. Just like the third, the fifth would be Robin regardless of what people said. So with a heavy heart, Dick Grayson, now Batman, took Damian Wayne, as his Robin and silently prayed that nothing would happen. But it did. Oh did it.