Donatello can't take it anymore.
For years now, he's been in an unrequited love with his hot-headed brother, Raphael. The pain that he suffered when he realized these forbidden feelings was unbearable, and when he knew those feelings would never be returned: devastating. But that pain is now an everyday ache, and he could have suffered happily in silence for the rest of his life if that meant he could be by Raph's side. But then a couple of months ago, he realized that while Raphael could make him burn, his eldest brother and leader, Leonardo, made him ache.
Leonardo was his leader, one to go to for a battle strategy, and the one usually open for suggestions. Though highly traditional, Leo could give Don an inner strength to strive to do better. His control on his emotions was incredible, and some days Don wished he had half the patience Leo had. And even though his feelings for this brother were relatively new, they hurt just as much as his long love with Raph, and knew even before he admitted them to himself, that these feelings would also be unrequited. But his growing emotion for his youngest brother was the straw that broke the proverbial camel's back.
Michelangelo was the one who could make a room brighter just by walking into it. His smile can make any situation look optimistic. And while his jokes may be corny, his pranks are well thought out and are designed to make everybody laugh. While he was clumsy in Don's lab, his grace and speed on the battlefield were unequaled. And though Mikey likes to cross-dress and might have been open for a relationship, he was still the youngest, and a bit naïve and shallow, and Don didn't want to go for a romantic relation if Mikey only thought it was just for a good time.
So here now Don was, activating a secret hypnosis trick that he did on everyone he knows and loves that he had hoped he would never have to use, especially after that trip to that terrifying future. But, he reasons, if they don't remember of my existence, how can they get hurt? Especially since I'm not going too far…
After he says the semiconscious sentence to activate the amnesia, he gathers everything he owns or has him in it, packs them into his Shell Cycle, and after a tearful whisper of goodbye, heads to new territory he had staked out a week ago.