Donnie was fine with scritches, that's why he had the shell washer after all. He could be in the comfort of a nice warm shower with a robot doing all the work, and nobody could hear the churning he found embarrassing while he from time to time gave himself something to relax with. (And sometimes some “relief”)

You didn't know about his much hidden obsession with the way it felt with scritches, until one day you found every little spot by complete accident. His chin, right under his jawline, and the small little spot right behind his ears. You two were relaxing, he was talking to you about some science documentary while it played in the background, and he was laying on your lap while you gently massaged his very much tense shoulders. And that's when it happened

You were rubbing your hands into a few different spots, like right on the edge of where his plastron starts at the top or up the back of his neck, but you gently dragged your nails under his chin and his mind melted almost immediately. He started to quietly churr in the middle of sentence, and that snapped his lips shut right away. So.. you did it again, and he held in every noise until it was too much, letting out each of his little churrs and chirps, looking up at you with lovesick, embarrassed eyes.