Plenty of blogs have tackled this subject (even I have, in this link here), so I won't bore you with it again. But today, I ran across a fabulous quote that tells WHY showing is so much more valuable than telling. It's because you want your reader to be active, not passive. You don't want to spoon-feed them information and tell them everything. You want to show them, and let them decide for themselves what's happening, how a character feels, why a character just made that decision. Let the reader participate, feel like he/she has some hand in the process.
Here's the quote:
"Don't give the audience 4. Give them 2 plus 2." ~Andrew Stanton, Director of WALL-E