Why did sir john a Macdonald want confederation?

Upper Canada was feeling the effect of the United States Manifest Destiny policy which would see Britain and all others displaced by the USA. British and Canadian, as well as Mexican, Spanish, French, even Russian land was being taken by the USA often by force.

Aligning and working with the remaining colonies Canada was hoping to present an effective military response to U.S. actions, which over the years had included invasions.

Much of the support and work towards Confederation was done by First and Second Generation Canadians, including those from Ireland and Scotland. In Canada these people are often called English or British but they were people who had been defeated by the British Empire and would not consider themselves English. Many were openly hostile to the British Empire.

Upper Canada also had the largest population of all the colonies, meaning they had the least to fear or lose from joining such a Confederation. They did manage to set up the ruling system in such a way that this larger population was translated to dominance in Confederation enabling Ontario to effectively rule over most other members. So much so that Ontario's history is now in many ways considered Canada's, the Confederations history.

Upper Canada had everything to gain and little to lose from a Confederation with other smaller colonies.