Hope

"Do not spoil what you have by desiring what you have not; but remember that what you now have was once among the things you only hoped for."

- Epicurus

Personal Take:
I have a tendency to think of the grass always being greener on the other side of the fence and, therefore, often have a propensity to not fully appreciate that which I already have.  This quote is so, so, so true and is one that I hope to keep in my mind the next time I start wishing for that which I do not have.