1. Because it's fun and
2. Because it annoys the 2-foots, especially when we do it during the night when they are trying to sleep.
Anyway, for the last week or so, Millie has insisted on a 2-foot carrying her downstairs first thing every morning, so I asked her why... She said "It's in honour of Dandy. He didn't do stairs, so the 2-foots had to carry him up every night, and down again every morning. It just helps me to feel close to him by getting them to carry me down every morning". She's a big softy, but I wouldn't swap her for anybun else in the whole of Lagamorphia!