Reach exceeds his grasp

What is the meaning/origin of the phrase:

(his) "reach exceeds his grasp"?

"Ah but a man's reach should exceed his grasp, or what's a Heaven for?"
It's from Browning, and it's a call to attemptthe impossible, the unlikely, the more-than-you're-comfortable-with. The attempt will allow you to gain the most. Don't settle for what's easily attainable.