Canadians also miss their cross-border travel, shopping, and entertainment trips. Just as the US has determined the need to protect its citizens from additional threat of Covid, Canada has taken that same stance. No different than the US or any other country closing its border to visitors from other countries.
At this time, I believe the decision is agreed upon by both Canada and the USA. Prime Minister Trudeau bases his decision primarily on input from the provincial premiers in conjunction with Federal and Provincial Health Ministers and Officers. Also, most of Canada's major and medium sized cities (Vancouver, Toronto, Montreal, Regina, etc) are in the southern parts of the provinces, so in fairly close proximity to the US border. An influx from the USA with the higher rates of infections could result in an increase in infections/deaths in Canada.
As per today's worldometer, infections per million in the US are 20,026, roughly 5 times greater than Canada at 3,587 ; deaths per million in the US are 596, roughly 2.5 times greater than Canada at 246.
Premiers Legault, Ford and Horgan (Quebec, Ontario and BC, Canada's 3 most populous provinces) are wanting the border closed until some time next year but PM Trudeau is reviewing it in 90 day increments in consultation with all provincial premiers.