For me it's going out socially in large groups, alone. That said, I went out, alone, to the rider meet and greet for the Boston Brain Tumor Ride last night. And what happened? I met Steve. A PhD student in Mechanical Engineering from MIT. But more personally, a first time rider who this past Fall had a tumor removed from his frontal cortex. He is signed up to ride 10 miles... when he admits he probably shouldn't be riding at all.
Talk about living at the end of your comfort zone.