"Happy" birthday

"Happy" birthday?

A common greeting passed every year.
Receive it once a year from a thousand over people, if you are popular
and say it a thousand over times to people, if you can remember and if you have that many people to say it to.

What is the definition of a "Happy" birthday?

If I choose to sulk on my birthday, would it be a happy birthday?
If I act happy on my birthday, would it be a happy birthday?

Must I be happy on my birthday?

Why do people associate Birthdays as Happy?
Why can't people say : Hey you, Birthday!? or Hey you! Norm Birthday!!

I wonder.....