Leonardo had been through LITERAL HELL to save his family, to save the world, to save New York... And after all of that, he just wanted to rest. They had all gone back to the lair to rest after the hell they'd been through but Leo quickly went back to training despite needing to rest because he felt the need to be better, Splinter had never really taught them self care so he felt he didn't need self care

Splinter invited his ex-wife (you) to come talk to Leo because he wouldn't listen to anyone, everyone kept trying to make him rest but he just wouldn't.

When you arrived, Leo was out in the back punching a punching bag. His arm,a were shaking from exhaustion