To me, the wardrobe is a symbol of something like a gift so small and dark, yet so much more once it was opened up, literally.

When Lucy first opened it and walked in, the wardrobe seemed dusty and full of nothing but coats and moth balls. Nothing extraordinary was seen. However, as Lucy advanced on, the world of Narnia came to her. Here was where the adventure would begin for the Pevensie children. Many lessons would be learned from Aslan Himself.

I'm not exactly sure about shutting the wardrobe/ Edmund, but what I think that meant was along the lines of great gifts should be shared with others. By Christian means, we should share God's love for us. Unlike Edmund, Lucy was wanting to share Narnia with Susan and Peter.

It has been awhile since I have read LWW, so all of this is based off of what I can remember.

-Why is it significant that Lewis continues to say "You must never shut yourself up in a wardrobe" if these observations are accurate?

Perhaps because by shutting yourself into the wardrobe you are refusing to acknowledge where you came from? Edmund denied that the problem was him: it was Peter, Susan, and Lucy who were the sources of irritation and discontent. Narnia was there; he was at the lamppost, but he looked for something else.

I am "gnawing" on the ideas you presented DiGoRyKiRkE, and I might come back and post something later.

However, I want to say that I think that Lewis kept saying that it is foolish to shut one's self in a wardrobe - because it is. Back then, there were so many true stories of small children who had died hiding in a refridgerator or some other common device that was impossible to open from the inside (and many wardrobes were like that - although they generally weren't airtight like a refridgerator is). I think that was just Lewis' adult sensibilities coming out - we tend to reinforce those kind of messages whenever we get the chance. (Look both ways before you cross the street, Don't talk to strangers, Don't take candy from strangers, etc.) I think that Lewis didn't want kids to read his book and then crawl in a wardrobe and shut the door looking for Narnia - and the parent not finding them until they were severely dehydrated or worse (if they had a wardrobe that had a catch on it that you couldn't open from the inside).

Even with that approach, it is significant that Edmund was the only one that closed the wardrobe door - he was of the mindset to completely ignore all the good sensable advice given to him from anyone. This made him much more willing to follow the Witch. We are the same, a lot of the time when it comes to the good sound advice we find in Scripture. We tend to think we know better - or that bit of Scripture was just for the people of that time (often when we do this, we don't understand the reason behind it and therefore don't translate it to comparable actions in our life today).

That being said, I will continue to contemplate your take on things and post again later if any thoughts worth sharing come to mind.