When World War I broke out in 1914, all Dominions of the British Empire, including Canada, were called upon by Great Britain to fight on her behalf. So, they kinda had to.
And start.ariot is right. Canada basicly had no choice. It was different in WWII, in which Canadian Parliament debated whether or not to go to war. Canada did, but the fact that she could make a choice was a major step in the growth of Canadian autonomy.