If she was the type of person to say something of that nature we wouldn't be as close as we are. "Lin Daiyu hears this through the closed door and is overwhelmed with both delight and sadness. Her delight is due to the fact that she thinks of him as her intimate friend, and on overhearing his speech knows that the feeling is mutual.
黛玉在門外聽了，又喜又悲，喜得是「素日認他是個知己，果然是個知已。」