Saying grace before a meal can help you maintain tranquility (thereby making it more likely you will experience positive emotions) via framing effects:

Before eating a meal, those saying grace pause for a moment to reflect on that fact that this food might not have been available to them, in which case they would have gone hungry. And even if the food were available, they might have not been able to share it with the people now at their dinner table. Said with these thoughts in mind, grace has the ability to transform and ordinary meal into a cause for celebration.

-- William B. Irvine, A Guide to the Good Life: The Ancient Art of Stoic Joy