"I'm in a good mood today," 为什麽要加in?im a good mood today不行吗?

"I'm in a good mood today," 为什麽要加in?im a good mood today不行吗? Today is my birthday,I am in a good mood I'm in a mood today. A very good mood today Not in a good mood today you wife I am in a good mood,bad mood I was there.Your attention to m its a fine day today is as good as my mood in the weather ma I know you're in a bad mood today. i'm in good mood now in a good mood,in high spirits in (a) good mood ,加不加 a Seems you are not in good mood today