Whenever the Doctor and Ditzy stop bickering and actual work together is just sweet.
During one puzzle, the Doctor and Ditzy are near the end...then Ditzy falls and dies. The Doctor is distraught at first, then instantly jumps and kills himself as well to get back to her. Ditzy's reaction is heart-meltingly adorable.
A minor one; when Ditzy starts (overly) freaking out from cat hair being in her face, the Doctor comes up to her and asks if she's alright, actually sounding a bit worried.