Citizenship has its privileges--in Canada, they include voting in provincial and federal elections, travelling under a Canadian passport (safer in some countries than under a U.S. one), and legal protections against security certificates and other "anti-terror" laws of dubious constitutionality. I've spent enough time in Canada to be eligible for citizenship, so why not take the plunge?
The answer is one my sig-other teases me about: I won't take an oath to "be faithful and bear true allegiance to Her Majesty the Queen".
It may sound silly, but I'm a republican in the old-fashioned sense of the word--I really believe in the virtues of democracy over monarchy, and I see Canada's ceremonial allegiance to the Queen of England as the last vestige of a system of government that is supported by neither reason nor history. If I were to swear to "be faithful and bear true allegiance", I would consider myself morally bound not to criticize the Queen, argue against the monarchy, or vote for a constitutional amendment to dethrone the House of Windsor (as almost occurred in recent years in Australia).
I'm not the only one who takes such an oath seriously: a class-action lawsuit is currently pending in Ontario, challenging the lawfulness of the citizenship oath on exactly the same grounds as I do. In Roach v. Canada (Attorney General), the Ontario Superior Court of Justice found there was sufficient grounds for the challenge to survive a motion to dismiss and the litigation continues.
To the question: it's just a silly oath--why does it matter?, I can only respond: if the government takes an oath seriously enough to deny me citizenship over a refusal to swear to it, I'll take the requirement seriously too.