It is certainly a good thing to be happy (when that happiness isn't making you harvest human intestines for food, for example). This goes back to the standard antinatalist "no one wants suffering, so let's reduce suffering, okay?". It's not going to do a WHOLE lot for the human race, reducing your own suffering that is, but it's still a good thing, just as preventing a stranger being mauled in the fingers by a piece of paper is a good thing, though paper-cuts are not a major cause of suicide.*
But happiness isn't all ice cream and orgies. Well, it is, but there's more to it than that. If you really want to reduce suffering in the world, you can't just go around throwing new humans into existence, despite how they may indeed be the 'ultimate expression of love', and how it may make you happy for the rest of your life (though it probably won't). You can't go around pillaging villages and killing monks either, because despite how you will be making both you and a whole lot of Vikings happy, you are causing suffering, and you are violating consent. Happiness is like a psychotropic drug. Well, actually, it pretty much is from the chemical perspective of one's brain. But as far as the analogy goes, happiness is great fun, and you can share it with your friends, and you can space out and start writing about things you won't understand when you aren't happy. But also like a psychotropic drug, it can make you do things you are seriously going to regret if you don't stay lucid, worry as much as possible, and remember to hold on to your most firmly-held beliefs as much as possible before you experience the 'trip'.
Like Hallucinogen Persisting Perception Disorder, you can experience unwanted mutations to your brain upon 'abuse' of psychotropic drugs (well, some of them). Happiness, on the other hand, results in, oftentimes, Happiness Persisting Perception Disorder, or as it is commonly called 'Always Looking on the Bright Side of Life'. What characterises this disorder? You begin to suffer from constant delusions - you believe that suffering is necessary for life, despite how you suffer increasingly less and less. You begin to feel that everyone is just as happy as you are, because you have forgotten what it means to feel horrifyingly normal. You believe that the future will fix everything - which could be true, but you do not consider the alternative situation that ending things right here, right now, would spare more suffering. Finally, in the late stages, you may believe that depression is completely fine, because there are a lot of tall buildings in the world.
Optimism is only really fixed by constant exposure to antitoxins - to real statistics and photos and books showing the constant state of suffering that the vast majority of the human race are in. Only then can you stop yourself making incredibly stupid choices. Only then will you finally have control over your reproductive organs.
Antinatalism is the radical notion that more suffering is wrong.
*I am not stating here that it is possible to become happy. If you don't medicate yourself, or meditate for an inordinate amount of time, the likely thing is, you will stay at the same happiness level you have always been at throughout your life (for example: lottery winners return to the same happiness level within a month or two of winning the lottery). Happiness is a Stochastic Phenomenon (thanks again, Sister Y).