Why the devoted believers of God keep on saying that they are happy?

Probably because we know that whatever happens, we can get through it by remembering that God is in control, and this (the full version):

Serenity Prayer

God, give us grace to accept with serenity
the things that cannot be changed,
Courage to change the things
which should be changed,
and the Wisdom to distinguish
the one from the other.

Living one day at a time,
Enjoying one moment at a time,
Accepting hardship as a pathway to peace,
Taking, as Jesus did,
This sinful world as it is,
Not as I would have it,
Trusting that You will make all things right,
If I surrender to Your will,
So that I may be reasonably happy in this life,
And supremely happy with You forever in the next.

How would you feel if you took death and hell as a basic living condition and everything else as a divine gift?

I'm not a perfect Christian, I'm not always happy, but the knowledge that all suffering and effort are for a virtuous reason makes me feel happier in general.

I challenge Nietzsche therefore: God is still alive!

Because religion is the opiate of the masses.

Merely political propaganda. I have never met happy religious types except child molesters.

Because they have attained spiritual peace...? I'm not sure. I have good days and bad days. :)

because their doctors put them on the good meds. kidding...so are you saying that they shouldn't be happy. or are you saying that non believers aren't happy?

Maybe because they are.

DELUSION.but some of us grow up..

I;m a devoted believer of God. I am happy. I won't keep saying it unless someone asks, though. You asked. So I'm saying.. I'm happy. lol..

Happy People Act Happy not tell everyone they are Happy.
But there is also A mind set. It goes like this FAke It till you make it. So sometime in the start of a path we must fake it and then we do make it.
Newbie Christians are same as Newbie 12 step people. As I am for 22 years. And then That path took me to the Lord
I am happy every day? Not at all. cause I do not take happy pills. But!! I have alot more happy days then sad depressed days. And when I have those bad days? I fake it till I make it. Then I say this big long Pray.... Like?
Thanks Lord for bringing me through another day. Humans are suppose to feel some do and some do not. Its more of a mental frame of mind. So there happy Jews at times. Happy Christians. Happy 12 step people. Happy Hindu, Etc... All people who walk in Faith have happier day.
We just have to saty out of the drivers seat

They do until something bad happens to them, they lose their smiles and drop out of worship so you don't see them anymore.

Because they are!

not all are. . . but to take your statement as true

people need to believe in something greater than themselves, be it God, or the gods, or ancestor spirits. thus if you have this one tends towards happiness.

deities often give purpose to ones life - that can make you happy. . . or not. . .

deities often chanel the divine too their followers - that makes you happy! :)

imagine, if one god makes you so happy - wouldn't four or five make you happier? *hint* ;)