I keep waking up! Help

Alright, it is normally VERY hard to wake me up from sleep- it requires two or more people shouting very loudly and sometimes sloshing cold water on my face to wake me up.
But recently I've noticed that when I'm Lucid Dreaming- the slightest sound, just somebody talking in my room, or the mailman outside shouting 'mail!' just wakes me up from sleep! I haaaaaate this, because it totally interrupts me in the middle of a lucid dream! I end up waking up abruptly repeatedly during sleep and cursing at such mailmen and people babbling in my room and going back to sleep, just to be awoken again...
But at the same time, I never am so easily disturbed into waking up when I'm dreaming normal non-lucid dreams.
Why is it that I'm so vulnerable to sounds when I'm lucid dreaming and how can I stop being this vulnerable and just sleep soundly though lucid dreaming?
Help!

I think the reasons are two-fold

1) When you lucid dream, and I don't know for a fact, but I know for me my REM sleep is a lot lighter than my normal dreams. And for this I am more susseptable to external noises etc and wake up more frequently. I am sure you can Lucid dream in heavier REm sleep - but I fell the lighter the better the chances and the increase in recall.

2) Because you are lucid dreaming your mind is more aware then it is when you are non lucid dreaming so because of this it is more aware of external sounds.

Adam's explanation makes a lot of sense.

You might try wearing earplugs when attempting to lucid dream. They won't block out ALL sound but they will muffle it a great deal.

I highly recommend "Joe's Earplugs" (http://www.joesearplugs.com). They're cheap but more than that they're COMFORTABLE and they work.

You can also go to Target, Walgreens, Eckerds, basically any grocery or drug store and get some ear plugs by "Mack's". They are made of foam and are shaped like small cylinders.