The people chose Sameness in order to control everything in their environment and their citizens, to prevent anyone from feeling discomfort.
At first, Jonas does not see anything unusual about Sameness. Things are just the way they have always been. However, when he begins training he sees snow, love, and war for the first time. He realizes that there is pain, but also greatness in what is missing.
The Giver explains that the choice to go to Sameness involved tradeoffs.
"Our people made that choice, the choice to go to Sameness. Before my time, before the previous time, back and back and back. We relinquished color when we relinquished sunshine and did away with differences….We gained control of many things. But we had to let go of others." (Ch 12, p. 95)
The Giver asks Jonas to consider what might happen if people were allowed to have choices, and Jonas realizes that they might make the wrong ones. He understands that there are benefits to having everyone’s choices made for them. He realizes that there are choices he wishes he could make, such as simple choices about what color to wear. He does not make that choice, because there are no colors.
Jonas accepts that people might make the wrong choices, and therefore it might not be good to allow them to make them. Yet Jonas and The Giver come to the conclusion that it is better to give people the chance to mess up than to never give them choices at all.
Lowry, Lois (1993-04-26). The Giver (Newbery Medal Book). Houghton Mifflin Harcourt. Kindle Edition.