why was Jack happy to see Jo switching her legs tensely and
Why jack felt being an old man suited him

Dear student,

Jack felt pleased to see Jo switching her legs tensely as that showed she was listening carefully and was interested in the next twist in the tale.
Jack felt being an old man suited him as he felt his imitation of an old man with a whining voice and a scrunched up face delivered the right impact while telling a story to Jo.

Regards