TMNT headcannons.

#I just love Donnie so much XD

Donnie wasn't the best fighter within the Hamato Brothers, he knew that and it always bothered him that he would be useless to them in the future.

One time when he was only eight he sparred with Raph during a training session only to be mercilessly defeated by the red masked brute. It wounded his dignity so much, and what made it worse was that Raph would always bring it up and make fun of him which ended with him screaming and throwing his staff at Raph's face declaring that he quits.

Leo, being the responsible one, decided to talk to Donnie (After giving Raph a lecture that he would never want him to repeat.) After hearing him out Leo suggested that he could be the scientist of the group, That he could also help him out with his staff if he had the time, After hearing that Donnie felt like he could actually play a big role in the group and took Leo's suggestions to heart.

And that Folks, Is why Leo is Donnie's favourite brother.