good fences make good neighbors; a hedge between keeps friendship green
Proverb meaning that even in close relationships, maintaining a certain distance or formality helps preserve harmony.
Even with close friends, as the saying goes, 'good fences make good neighbors,' so it's better not to be too presumptuous.
A variant of the proverb 親しき仲にも礼儀あり, “even among close companions there should be courtesy,” with 近しき representing the older adjective form.