The Doctor checks his reflection in the mirror, disappointed; straight hair again. This is his third regeneration with mostly straight hair; how long has it been since he had curls? It’s not completely straight this time, there’s a bit of a wave to it, but it’s not curly!
Straight hair is boring, just sits on top of his head, not doing anything interesting, and then flops in his eyes when he’s trying to do stuff. That’s why he kept it short the last couple of times. Curls stay put, and sometimes they bounce. He can’t wait to be curly again.