If not you, who? If not now, when?
A single candle can light a thousand more without diminishing itself.
The shy man will not learn; the impatient man should not teach.
Moreover he saw a skull floating on the surface of the water and he said unto it: Because you drowned others they drowned you; and those that drowned you will eventually be drowned.
Judge not thy friend until thou standest in his place.
Take care of yourself - you never know when the world will need you.