Promising an immigration officer to return is useless, and probably counterproductive. The way to satisfy them is for your course of study to fit in well with your past, your interests, and your plans. It shouldn't look like a kamikaze mission in which you are burning all of your resources and all of your families resources in order to qualify to apply for CEC permanent residency. Explain your education and work history, say exactly how your course of study fits in with them, and describe what you will do with your Canadian degree in your home country after you've finished. Explain why a local degree isn't as good, and why you've decided to spend the extra resources to study in Canada. Don't talk about 'lifelong dream to study in Canada but staying has never crossed my mind', talk about what you've done and what you hope to do and why Canada would fit well as a part of it.