It's not true that it costs nothing to be kind.
It costs self-reflection. It costs a moment to pause and consider. It costs taking the time to listen and understand what's needed in the moment—which means taking in the whole situation, and letting go of your own projections.
Kindness is radical.
Kindness with healthy boundaries is more so.
Kindness isn't free and we can't always spare the price. But it's worth it when you can practice coming back to it day after day.