Bhagavad Gita, Chapter 2, Verse 66

There is no wisdom for the unsteady, and no meditation either. For one without meditation there is no peace, and how can one without peace be happy?

Without some inner steadiness, real peace isn't really on the table. And without peace, neither is real happiness.

Krishna draws a direct line here: an undisciplined mind can't hold a clear focus, an unfocused mind can't find real peace, and without peace, happiness stays out of reach no matter how good the circumstances look from outside.