Could be a couple things - but is it really that important as long as you're becoming lucid?

Sounds like being alone acts as a dreamsing for you. My guess is that when you're alone, you're more likely to think of your 'self', becoming more conscious of what you are doing, where you are, etc. So, the same goes for deams.

When you are around other people, you're distracted and too busy focusing on THEIR actions and existence. It's when you're alone that you're more likely to focus on your own status, "where am I, what am I doing, why am I alone, what am I looking for... am I dreaming?