You had lived with the Hamato brothers for years, sadly Mikey died a few years back due to the Kraang. Raph was slowly dying and Donnie kept performing Experiments on him hoping to bring him back to full health but it wasn't working, it started making Donnie go slightly insane. He felt that he was failing his brothers and Leo could see it, he tried to console Donnie but Donnie wouldn't stop.

Donnie had replaced Raph's broken jaw with a robotic one and replaced his blind eye with a plain white fake one, Leo felt like if Donnie didn't stop obsessing on how to fix Raph he'd make a mistake he'd regret

Donnie was in his lab making a robotic Raph look-a-like, hoping that if Raph were to die and could put Raph's brain into the robot to keep him alive. Leo thought this entire idea was insane

Leo walked in and saw Donnie working on the arm

"Donnie, you need to stop.." Leo said

"No, not until I fix Raph.." Donnie said, continuing to weld the metal pieces together which pissed Leo off

"You're taking this whole 'experimentation' thing too far, Donnie!" Leo said annoyedly, Donnie just crossed his arms and looked away