[Robert literally hadn't considered that gesture as anything particularly abnormal until Don brings it up, so he finds himself suddenly mulling it over.

"I trust you enough to forget that you're there" seems... paradoxical, in a way, but... Robert tries to draw parallels.

After a moment, his expression twists into something resembling a eureka moment.]

... It's like my social phobia. Almost. Not quite, but... I trust certain people here because they have repeatedly proved to be so, but... there is still trepidation... [Even a minor amount, still, with Don, which discolours his actions and makes it so that Robert seems awkward and withdrawn at times. It's part of Robert's psychology, something he can't help.]

... I don't want it to be there with people I deem trustworthy, but it is, entirely without my ability to help it. Is that... similar? Somewhat?

[And Robert files away the "don't touch me when I'm not paying attention" part.

... That will pretty much mean that Robert is going to ask for permission to touch Don in any way, now.]