I haven't stayed as constantly in touch with Mark as with a few other camp friends, but he was a good friend back in the day. I feel bad for his family, whom I've never met. (Not counting his brother and sister, both of whom were also at camp with us.)
Of course, this also inspires me to keep up my efforts to stay reasonably healthy. People our age aren't supposed to have heart attacks, and I'm hoping my efforts to eat fairly healthily, along with my cholesterol meds, will keep me relatively safe from that kind of surprise ending.