why did the united states, canada, and most western european countries form the north atlantic treaty organization?

To protect each other against communist nations military attacks

Explanation:

The united states, Canada and most western European countries did not practice communist kind of Government unlike the soviet Union who was largely a communist community.

The Communist countries which were Eastern European countries and the Soviet Union attacked most countries not practicing Communism in a bid to enforce communism in those countries. The NATO was formed by allies of democratic government to protect each other collectively from the attack of the communist communities when they try to enforce communism on democratic governments.

I believe the reason why the western world had formed the NATO alliance was because it served the purpose of protecting westwern europe against the influence of the Russian Iron Curtain. It was a pact that brought all of the US's allies together into one alliance to protect against Russia and any other countries that were supporting communism at the time.