[It's about happiness and, more specifically, her happiness. If that includes protecting her cat from potential cat burglars then that's exactly what he'll do - it's by no means the biggest risk he's ever taken and the reasons behind it are the same as always. It's what you do for your friends, no matter if it's stopping civil wars or taking down God or vigilant cat-sitting!
That soft smile of hers is all the reward he needs, and he returns it easily before following her inside.
It's very pink. Very, very pink. It's also very much Miwako and that allows him to appreciate the pinkness of this very pink pink room. He may even be giving a low, impressed whistle as he follows her inside, setting the sewing machine down on the floor.]
I'm impressed, Miss Miwako! You've really made yourself at home~!