I think that's typical of people who have a lot going on in their lives and are always required to devote their time/attention to someone else. If they were suddenly faced with some personal freedom, they wouldn't know what to do with it. They never developed any hobbies/interests, because there simply wasn't time for that - so they feel like you must be terribly bored if there are no duties/obligations to fulfill (even going out with friends is an obligation, to an extent, because you may not necessarily feel like going, but you don't want to disappoint them).

Or it could be just the eternal extrovert/introvert dichotomy. I've had people ask me questions like: "What can you possibly do for hours at home all by yourself?..." - and I honestly didn't even know how to BEGIN aswering that.:P