It's when you trust someone, and they can trust you. When you make each other happy, when you enjoy and feel comfortable in their company. And when you would do anything* to protect that.
Alternately, there's the sort of the love my siblings and I feel for each other. We may not necessarily get along, but we've got each other's backs, and we understand each other in a way that nobody else does.
*My pedantic mind wants me to note that maybe not anything. Moderately extreme things.