the secret of happiness?

[Eway]

Dear Teachers,

I read this from The Tenderness of Wolves by Stef Penny:

"...the exhausting journey wearing him down...so the sight of the trading post is cause for jubilation. At this moment, Donald can think of nothing more glorious than lying down on a bed for a very long time. Clearly the secret of happiness, he reflects quite cheerfully, is a variation on the general principle of banging your head against a wall, and then stopping."

Why is the secret of happiness lies in "banging your head against a wall, and then stopping"?

[Anglika]

Think about it!

You bang your head against a wall repeatedly. It is very painful. Then you stop and the pain stops. Phew! What a relief.