Don't know why we couldn't have gone swimming this afternoon. Is it grammatically correct sentence?

No because it is incomplete. You would need "I" at the beginning for it to be grammatically correct.

Explanation:

judy jones is an attractive woman who receives attention from many men throughout the story.   despite her beautiful appearance, she is superficial and callous.   she is an aggressive woman who goes after any man she desires but she is not happy.   she is fickle and is never satisfied.   her beauty is only skin deep.   dexter breaks down because he knows his dreams are unattainable.   he believes that wealth and physical beauty are what will make him happy.   he soon realizes that she has lost her attractive looks.   it is not all what he had planned.