This is somewhat related, but i have literally never met a single ADHD adult who wasn't the chillest person ever. I suspect that a lifetime of learning to go easy on ourselves and set reasonable expectations for ourselves transfers pretty well to being patient and kind with others.
I think ADHD often does to us sort of what some other conditions do to others: beats us down. By the time we reach adulthood, we've learned from millions of experiences not to bother with certain things. At the same time, many adults I know with ADHD are much more anxious, especially in social or work situations, than they appear.