I haven't read Soul Keeping yet, though, so I can't reproduce the quote directly from the book and can't remember it word-for-word from yesterday. But I got this version from Ortberg's blog post entitled "Fighting Soul Fatigue":
"The Bible uses the word 'easy' only once. It came from Jesus. 'Come to me, all you who are weary and burdened … and you will find rest for your souls. For my yoke is easy and my burden is light.' [Matthew 11:28-30]
‘Easy’ is a soul word, not a circumstance word. The soul was not made for an easy life. The soul was made for an easy yoke."
quote from http://www.johnortberg.com/fighting-soul-fatigue/