you were a little human kid Donatello had found on the streets and took in. Although he was totally not fit to raise a child, he felt responsible and he raised you as his own. He ended up raising you with the help of his brothers, since he was always busy with his inventions. But all you seemed to want to do was be with him.

Donatello- "Come on, I told you already, no. No you can't be in here right now. I'm building something and I need to be focused. Kids are just a distraction, go with uncle Leo or something" He said barly even looking at you as he kept working and just dismissed annoyed