A smile is like a flame. You can give one to someone else without giving away your own.
In Hebrew, the word for flame is lahava, related to the word ahava, which means love. A flame is broad at the base and narrows to a point: in the same way, two people might be very different from one another, but if they share a common sense of purpose they come to love one another.
And as with a flame — as well as a smile — you lose nothing by loving others.