004. Insides
005. Outsides
As a Dark Master, it was very important for MetalSeadramon to know what feelings to show and what ones to keep to himself. Showing the wrong emotion in front of Piemon could mean the difference between life and death.
Of course, it could be difficult to do that when one of those emotions that you had to keep inside welled up so strongly inside of you whenever you saw one of your teammates.
MetalSeadramon had always been good at controlling his feelings, but ever since he and Mugendramon had had their "heartfelt confession" moment in the Master Chamber, every time he saw his partner, he felt a strange desire building up inside of him that wasn't always that easy to control. Luckily he had trained it so that, at the very least, Piemon didn't notice. But it always made it that much more difficult to see Mugendramon ignore him in order to keep both Piemon and Pinnochimon in the dark.
Of course, there were a few occasions when he slipped up, and Pinnochimon became suspicious. He never fumbled when around Piemon, at least, but if Pinnochimon were to find out, it would be just as disastrous. Dark Masters were supposed to be stoic, unfeeling machines. They weren't supposed to fall in love.
And, of course, when the were alone, all the barriers they'd put up to keep their fellow Dark Masters off of their trail would come crashing down. When they were alone they didn't have to be Mugendramon and MetalSeadramon, two of the most feared, ruthless Digimon of all time. Instead, they were free to be Mugendramon and MetalSeadramon, the two irrevocably in-love Digimon that just happened to be Dark Masters.
And when they fell back under Piemon's watchful gaze, they kept up appearances.
Because it was important to be able to differentiate between insides and outsides.