Does Halloween Oppose the Economic Principle of Self-Interest?

Why do people but candy with their own money and give it out at no cost? Spending money for strangers doesn't follow the principle of rational self-interest. Then again, why do people donate or buy presents? Their festive spirit may be the driving force in them giving out candy at no cost. I get why people walk around asking houses for candy, it's free candy! But I believe in order for there to be Halloween, everyone needs to contribute and be active in the community.