But nothing can take the place of love.  Love is the measure of life: only so far as we love do we really live.  The variety of our interests, the width of our sympathies, the susceptibilities of our hearts—if these do not measure our lives, what does?  As the years go by, we are all of us more or less subject to two dangers, the danger of petrifaction and the danger of putrefaction; either that we shall become hard and callous, crusted over with customs and conventions till no new ray of light or of joy can reach us, or that we shall become lax and disorganized, losing our grip upon the real and vital sources or happiness and power.  Now, there is no preservative and antiseptic, nothing that keeps one’s heart young, like love, like sympathy, like giving one’s self with enthusiasm to some worthy thing or cause. 

--John Burroughs