Because... he just does. Joe is constantly, distractingly aware of it.
+1 for Nicky absentmindedly chewing on pencils/sucking on straws/whatever mouth related shenanigans you feel like without realizing the effect he's having on Joe. +2 for someone else absolutely realizing the effect Nicky's having on Joe and being vastly amused by it.
Joe/Nicky + Nicky Has a Pretty Mouth
+1 for Nicky absentmindedly chewing on pencils/sucking on straws/whatever mouth related shenanigans you feel like without realizing the effect he's having on Joe.
+2 for someone else absolutely realizing the effect Nicky's having on Joe and being vastly amused by it.