I think that one of the main messages of The Giver is that anyone has the power to make societal changes. Jonas was a change-maker because he sacrificed himself for the sake of his community. There have been many real people who did the same thing in our world. As the saying goes, individuals are the only ones that can really make a difference. If you see something wrong, it is your responsability to do something about it. Start small, but do something.
I think that it is not too hard, too much of a stretch to believe that a society would want to get rid of suffering and troublesome emotions. While it isn't very organic, these people have made a choice to control things and keep everyone "comfortable." If you make connections to our own society, think about how we use drugs to alter our moods -- there are drugs for attention, depression, all sorts of mood-altering substances that our society uses to control our feelings, not really all that different than what Jonas' society does (though they take it much further). I think there are many examples of this sort of connection that you could draw parallels from our society to Jonas' society.
As for who had made the decisions to make these changes, the history is not ever explicitly stated, but the community is run by Elders who act as a sort of government for the people. It seems as though this sort of change (getting rid of colors, emotions, etc) would have happened over a long period of time, probably slowly laws were enacted and new technologies became available that allowed people to become more "comfortable."
Obviously we aren't told directly what "happened" in the past to create the society that is depicted in this novel. However, we can detect clues from the way that the society is and what they have tried to erase or stamp out. It is interesting that suffering is one of the key elements that has been "deleted". Also note that the "feelings" that teenagers experience when they undergo puberty are controlled by medication. As pohnpei397 suggests, perhaps humanity at this point in history had experienced another major world war or major discrimination. This could have led the leaders to take a very radical move to prevent such an event from happening again and to create their idea of a utopian society. Of course, it seems the novel suggests the only way of creating a utopia is to take away freedom of choice and will and oppress certain groups, thus making a dystopia.
Given the other question asked about The Giver this morning, and given that you say "Connections" here, I wonder if you are not talking about what actually happens in the story.
I am going to assume you are asking how their society could change to get from a "normal" society to what they have. I would say that this could only happen by having the people really buy into and agree to the changes that are being made. The changes could not possibly, in my opinion, have been forced onto the people by the government if the people did not like them.
Changes like this must have happened in response to some sort of outside forces. Maybe there were too many wars. Or maybe there was a lot of hatred within their society or a lot of poverty. When there are crises, drastic changes can happen and people will approve of them (like the Nazis coming to power).
So I think that sometime in the past there must have been things like that which made the people want change.
In the book The Giver change occurs very seldom. The whole idea of the existence and function within the community is not to have change so that the people will have no confusion of the rules or circumstances in their lives.
At first appearance it seems that the elders are the ones who have he power to initiate change in the community but their own thought processes are limited. They must go to the holder of the memories (the Giver) who has all the memories within him to ask what changes should be made in unfamiliar circumstances. The elders then meet to determine if changes need to occur.