Child is afraid of being alone

How should we handle our eight-year-old son's strong aversion to being by himself? He sleeps in the same room with his younger sister because he doesn't want to be alone. He won't play in his room unless someone else is there. The last time he tried, he suffered a panic attack (sweating and shortness of breath). He's very intelligent, does well in school, plays piano, drums, and soccer, and has many friends. He has never experienced any kind of trauma and my wife and I have a good relationship. What's going on?