Why not be totally changed into fire?

Abbot Lot went to see Abbot Joseph and said to him:
"Father, as far as I am able, I keep my little rule; I fast, I pray and meditate; I live in peace and according as I am able to I cleanse my heart of thoughts. What more can I do?"
Then the elder stood up and in reply he streched his hands toward heaven, and his fingers became like ten lamps of fire. He said to him:
"Why not be totally changed into fire?"

from The Wisdom of the Desert by Thomas Merton