What is the meaning of [Grasp all, lose all]

Don’t be avaricious. If you are too greedy you will lose even what you already
have. The dog in the fable was carrying a piece of meat across the bridge. He
glanced down into the water and saw what he took to be another dog with a
piece of meat in its mouth. He opened his jaws to snatch at the meat in the
reflection – and his own piece fell into the water.