I wish people could attain a constant state of happiness but that's impossible. With every up there is a down. I suppose you could remain neutral at 0. =/
Eternal happiness isn't impossible, it's just that here on earth we are unable to stay on a constant rise of vibrational frequency due to the negative energy surrounding us and the nature of man's mind. You can go for quite long though, that is something I have learned by falling in love. We can create a positive feedback loop that has no end, this would be the singularity/ascension of mankind.
This feedback loop would be carried into the afterlife and beyond, there is infinite love to be felt, and having felt infinite peace for a brief period of time, I can tell you there is no better feeling in the world.