The refund illusion
A big refund feels like a win and is actually an interest-free loan to the Treasury: you overpaid all year and got your own money back in April, without interest. A small refund (or small owed amount) means withholding matched reality — the correct outcome, even though culture treats it as a failure.
The two dials that matter
The W-4's real levers are extra withholding per period and the dependents/other-income lines. Under-withholding from a side gig is fixed by adding extra per check; a working spouse's combined income gap is fixed the same way. The goal is a refund near zero, not a lottery ticket in April.
Life events reset the dial
Marriage, a second job, a spouse's raise, unemployment mid-year — each changes the annual tax while withholding keeps running on the old assumption. A 10-minute check after any life event (or a mid-year paycheck comparison against the take-home calculator) prevents both the April surprise and its opposite.