When you can be together, not say anything, and just know what they're thinking. Or even when you can not say anything to each other and have it mean just as much as if you said something.

When you love them not despite their "flaws" but because of them.

When you can call them out when they're being bitchy or on how their acting (and vice versa) and then settle it without creating drama.

I could go on but I really don't care.