ISTJ's attitude toward friends is not simply tolerance, but a deep understanding based on a clear recognition of human nature. They assume that selfishness and self-interest are inherent parts of human nature, so they don't hold you to unrealistic "saintly" standards. When you reveal your darker side, their reaction isn't "Oh my God, how could you be like that!", but rather "That's normal, most people think this way, you're already doing very well." This kind of "taken-for-granted" attitude can bring a great sense of relief.