Robby regressing to a teenage age-range when having rampantly changing emotions like that of a teenager.
Dennis noticing and giving him space to engage in his 'teenage activities.' Giving Robby the time to re-read the books he enjoyed when he was 15-18 (mostly Vonnegut, all the rage among the vaguely pretentious and vaguely grungy teens he once associated himself with).
Practicing guitar, never seriously enough to learn anything new, only plucking at the strings to feel like a kid passing the time with a favorite hobby. Maybe playing the riff of some Radiohead song he learned from a friend in high school that has stuck with him still.
Dennis gives him space...up until a point.
Dennis hears how his voice changes in these moments, more fragile— breaking constantly due to a swelling Adam's apple or just general teenage insecurity. But when Robby's voice shakes more than it should while regressed, Dennis knows it as a plea for company in this state—even if Robby is actively being mean via that cracking voice.
It's only a sign for Dennis to stay near Robby, proving to his teen that he is worth the effort of being loved even while he is being difficult. The more that Robby pushes, however rude his insults towards Dennis are, it's only to test the strength of Dennis' pull.
Dennis will do anything to prove to Robby that he will never be abandoned again.
