I have entered the US on a Canadian passport from some other country (eg South Africa, Chile, etc) on more than one occasion. The decision to admit you or not is based on your citizenship, any US visas you have already applied for, your relevant travel history (eg you have been denied admission to the US in the past), and your stated reason for this visit along with whatever information, paperwork, and possessions supports or denies that stated reason.
I won't go so far as to say it's irrelevant where you're coming from. It's possible that coming from some places could lead an officer to suspect you are not coming for your stated reason. But it's awfully close to irrelevant. There is not a rule that says "admit if coming from X but not if coming from Y" for example.