When someone never leaves things that are already well. Taylor swift use of this expression on her single "ME!". She says: "I never leave well enough alone" she means that she always wanna improve things that are already well.
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.