His comment that he didn't get groups really resonated with me. I felt exactly the same way he did, that I didn't understand how to have more than one friend at a time. If two people you like are present at the same time, how do you connect with both at the same time? I never got it and still don't. If anything, it's gotten worse over time, and I think it's actually developed into some major social anxiety. Fortunately I'm relatively happy with how things are so I'm not particularly bothered about it.
I don't know that people are necessarily born introverts; I think one's early experiences are as big a factor. I think as a very young child I was pretty extroverted. But I was an awkward child - fat, crooked teeth, thick glasses, long greasy hair, garage sale clothes - and frankly most of my peers were pretty mean. I think this led to a general lack of trust and consequently a disinclination to try to make friendships.
I don't know what Robert's excuse was! :)