1. Canadian Nationalism. If we want to build an actual nation instead of a collective living arrangement of separate ethnicities and religions then the people residing here need to have a stronger allegiance to this country than religion or the country of their birth, their parents birth or even the countries of their grandparents and great grand parents. If the conflict overseas is so important to you that you need to leave here and fight then you should have to renounce your citizenship to participate or face the legal consequences when you return. While we're at it we should do away with dual citizenship for many of the same reasons as well.
2. Hypocrisy. As a country we have no problem expressing our collective disgust when some Muslim fundamentalist goes off to murder in the name of his god in conflicts that Canada is not involved in because at the core of our ethics we know that his involvement perpetuates these conflicts. There is fundamentally no difference in people personally going off to fight to promote their values as there is in people going off to fight to stop them from promoting those values as that violates their own.
3. Humanity.Most nongovernmental conflicts today gain traction, are extra violent and maintain their longevity due to the financial support of foreign Diasporas through the aid of money, manpower, equipment and logistics. When we allow organisations and people to operate from Canada in an attempt to destabilise other countries, regions, enclaves, or ethnicities we are tacitly compliant in violating the principles of humanity that Canada outright boasts it promotes and by extension have a part in many of the global conflicts and terrorist acts.
4. Rule of Law. States wage war. Private citizens do not. If we truly value democracy as driving force for peace then allowing our citizens to make their own choices about which war they will engage in goes against these principles. Our parliament must debate laws and decide what rules Canadian citizens will live under and if/when they can violate those rules. For example, currently it is against the law in Canada to commit murder (with specific legal exceptions for police soldiers etc). If a Canadian murders a homosexual in Thailand because they don't like gays and returns here they will be prosecuted. Likewise, if a Canadian in Thailand rapes a woman or has underage sex they will be prosecuted under our laws. Why should the above three crimes be given a pass when done if that Canadian is not a tourist but rather an agitator?
Well that's all fine and dandy, but who are you to police what I do when I leave Canada?? There are plenty of Canadians that have dual citizenship's and conflicting localities.
Many Canadians joined the US military to fight in the Vietnam war.