First, Trudeau really deserves "great". he cracked down the Qubec separatists and fought for the unity of Canada, and he intruduced the new Canadian constitution, and the Charpter of Freedom and Rights as well. He was one of the greatest Canadian PMs, with highest reputation in Canadian, which can be seen from the memories all over Canada when he died.
Second, when we use the word "Great", it shows our appraisal and respect to such a great stateman that China really need right now.this lets normal Canadians know what kind of Canadians have a good image in foreign countries.
Third, Canadian Jacks and Jennifers would be happy to hear we respect and appraise their national hero and more inclined to accept what we are saying.