The short version: it's air, not water, that's the problem
Gurgling happens when air gets trapped and forced back through a drain line instead of venting properly. Your plumbing system relies on a network of vent pipes (usually running up through the roof) to let air in and out as water drains — without that airflow, water draining in one place can pull air backward through a nearby fixture's trap, and that's the gurgle you hear.
Why it's often not the sink that's actually the problem
If your kitchen sink gurgles when the washing machine drains, or your bathroom sink gurgles when the toilet flushes, the noisy fixture usually isn't where the issue is. It's more often a partial clog or blockage further down the shared drain line, or a vent stack that's obstructed (birds' nests and debris in roof vents are more common than people expect).
When it's worth paying attention to
One occasional gurgle isn't an emergency. A gurgle that's become frequent, that's paired with slow drainage anywhere in the house, or that comes with a sewer-gas smell is worth taking seriously — those together usually mean a partial blockage that's getting worse, not better, on its own.
What we actually do about it
We don't just snake the loudest fixture and call it done. Gurgling is a system symptom, so we trace it back — checking for a blockage in the shared line and confirming the vent stack is clear. A camera inspection on a stubborn or recurring gurgle usually finds the real cause in one visit instead of guessing fixture by fixture.