These two concepts, happiness and freedom, can be understood as representing safety in numbers and danger/self-reliance.
The situations that characterize happiness and freedom, when seen from a certain angle, have everything to do with the individual's relationship to his community.
This is a fairly common theme in dystopian novels; I suspect that Orwell may have seen it in Zamatan's "We" and repeated the theme. From "We":
"Those two in Paradise, they were offered
a choice: happiness without freedom, or
freedom without happiness, nothing else.
Those idiots chose freedom. And then what?
Then for centuries they were homesick for
the chains. That's why the world was so
miserable, see? They missed the chains.
For ages! And we were the first to hit on
the way to get back to happiness."
There is a similar theme is Doestovesky's "Legend of the Grand Inquisitor" if you'd like to check into it.
I suspect that many people would give up freedom, but I don't like the word happiness; I think they'd give it up for security. We may have seen this after 9/11; we may see it now in our movement toward cradle to grave care from our government.
As another writer suggested above, freedom is difficult, but it is my feeling that only through freedom do we get creativity, a chance to realize our potential. I hope we select the difficult, but fear we may not be up to it.
I'm not sure what sources suggest that the bulk of mankind prefers happiness over freedom. How many men or women are truly free anyway? I have bonds to my husband, my children, my job, my pets, my house, my educational commitments. These things bring me great joy. By no means am I free, but it is this very lack of "freedom" that brings me joy. Perhaps you should clarify what type of freedom you want us to address.
I'm not sure they do, either. If and when they do, though, there are probably many reasons for doing so. Let me posit what seems to me to be the most likely reason people might choose happiness over freedom--happiness is easy and freedom is hard. What I mean by that is the single-minded pursuit of happiness requires little thought or responsibility; by nature, it's selfish and self-absorbed. When you don't have to worry about anyone else, life is easy. Not better, just easy.
Freedom, on the other hand, always costs. Someone had to fight to get it, and someone has to fight to keep it--because someone always wants to take it away. Freedom is hard because it generally requires tolerance of others' rights, even when that may interfere with your happiness. Freedom is the hard choice, but without freedom there can probably be no happiness--and too many aren't willing to make the necessary sacrifices for freedom.
The quote comes from page 152 in my edition of 1984:
He knew in advance what O’Brien would say. That the Party did not seek power for its own ends, but only for the good of the majority. That it sought power because men in the mass were frail cowardly creatures who could not endure liberty or face the truth, and must be ruled over and systematically deceived by others who were stronger than themselves. That the choice for mankind lay between freedom and happiness, and that, for the great bulk of mankind, happiness was better. That the party was the eternal guardian of the weak, a dedicated sect doing evil that good might come, sacrificing its own happiness to that of others. The terrible thing, thought Winston, the terrible thing was that when O’Brien said this he would believe it. You could see it in his face. O’Brien knew everything. A thousand times better than Winston he knew what the world was really like, in what degradation the mass of human beings lived and by what lies and barbarities the Party kept them there. He had understood it all, weighed it all, and it made no difference: all was justified by the ultimate purpose. What can you do, thought Winston, against the lunatic who is more intelligent than yourself, who gives your arguments a fair hearing and then simply persists in his lunacy?
So Winston, as the above editor astutely points out, is anticipating O'Brien's arguments about the common good of the people from the ruling party's point of view. Remember, the ruling party is a bunch of bold-faced liars, propagandists, and torturers. The book is a satire of a dystopia and totalitarian government. As such, the truth is the opposite of what they say.
Happiness is not chosen over freedom: ignorance is. Ignorance is not so much chosen either; it's only what's left by default.
Remember the slogan? "Ignorance is strength." Remember the first chapter of Goldstein's book?
THE THEORY AND PRACTICE OF OLIGARCHICAL COLLECTIVISM by Emmanuel Goldstein
Winston began reading: Chapter I Ignorance is Strength
Throughout recorded time, and probably since the end of the Neolithic Age, there have been three kinds of people in the world, the High, the Middle, and the Low. They have been subdivided in many ways, they have borne countless different names, and their relative numbers, as well as their attitude towards one another, have varied from age to age: but the essential structure of society has never altered. Even after enormous upheavals and seemingly irrevocable changes, the same pattern has always reasserted itself, just as a gyroscope will always return to equilibrium, however far it is pushed one way or the other.
The aims of these groups are entirely irreconcilable...
So, the people don't have much a choice, do they? They don't choose anything really, whether it be happiness or ignorance or freedom. From the government's angle, it's all about control of the masses and the death of the individual. The masses don't suffer from thoughtcrime: individuals do. The masses don't rebel: individuals do. The masses don't get tortured: individuals do. The masses are ignorant, and ignorance is bliss. So, it's easy to control and manipulate and deceive the masses, but it's harder to do the same to an individual.
I do not know that it is clear that the bulk of mankind does choose this. This is just what Winston thinks O'Brien will say. This implies that this is the party line -- that the Party officially says that they make people happy and people prefer that to freedom.
Now, assuming the statement is true, why would people do this? To me, you can argue that many people prefer comfort to the ability to speak their own minds, for example. This is because material things are more immediately important to most people -- they have more impact on the people's day to day lives. You can see in China that many people there are happy enough to be censored as long as they can get richer. People in Singapore are happy enough to accept all the rules their government imposes so long as they have wealth.
I think to many people, material comfort is more important on an every day basis than freedom of speech is.