1. I can't understand why...:
a sentence pattern used when angry or surprised at something or someone
- I can't understand why people spit on the street. It is so dirty.
- I can't understand why my boss makes us work late every day. It is unnecessary.
2. at all:
a phrase used to emphasize a negative sentence
- I haven't eaten at all today.
- He doesn't help us at all. He is useless.
3. to not have a sense of humor:
to not laugh a lot and to not understand jokes and why things are funny
- He is too serious. He doesn't have a sense of humor.
- Mary is not fun. She doesn't have a sense of humor at all.