It's Friday...

Why do people say that when it's obviously Friday?

For instance, I was walking down the hall today and someone said,
"good morning, how are you today"?
I replied, "I'm great, how are you"?
he simply said- "it's Friday".

I know he is relieved that it's the last day of the work week. But still, "it's friday"? It would have been slightly better to say, "I'm doing well, and it's Friday". I don't understand why people making the statement of what day it is, is supposed to be some kind of cryptic message of what kind of mood they are in. And this happened on Friday morning.... He still has the whole day to work, once again I can understand that statement better if it were the end of the day.

I used to work with a guy that predictably every single day he'd tell you what day it was in his greetings. It became an inside joke to me, that I'd make sure to say "hi" to him everyday just to hear what day it was. I believe everyone knows someone like this...

"Not too bad for a Monday"
"Oh, it's Tuesday... You know how it goes"
"Well, it's Wednesday, we're halfway there"
"I'll be better when it's Friday, at least it's Thursday"
"Yay- it's Friday, got any plans for the weekend"?