One of the Wisest Things I've Heard

"The hardest thing in the world to learn is when to stop." 

As in:

• Enjoying food is nice, but not when we eat too much.
• Enjoying someone's company is nice, but not when we cling on to them.
• Enjoying power is nice, but not when we sacrifice all values to maintain it.

While enjoying something, to have the presence of mind to know that the enjoyment will end if we abuse it is a very difficult thing to learn indeed.