Mono: what's my 'one thing'?

The 'mono' in monastic means "one thing", to be so focused on one thing that everything else fades away. ~ Singleness of intent, solitary purpose.

Wasn't there mention of this concept in that City Slickers movie with Billy Crystal? I think the language was bad in places, but the underlying philosophical message was correct.

What is my one thing?
Jesus?
God's glory?
Getting in?
Being safe?
Being comfy?
Serving?

Sometimes my motive or means get confused with the 'one thing' and become another thing.