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.