In Louis Sachar's novel Holes, why does Zero like digging holes?

There are several clues in the novel that suggest Zero has mixed feelings about digging holes. In an early scene, Stanley notices that despite being the smallest boy at the camp, Zero is the fastest digger. Not only that, his holes are perfectly dug:

Stanley walked to Zero's hole and watched him measure it with his shovel. The top of his hole was a perfect circle, and the sides were smooth and steep. Not one...

There are several clues in the novel that suggest Zero has mixed feelings about digging holes. In an early scene, Stanley notices that despite being the smallest boy at the camp, Zero is the fastest digger. Not only that, his holes are perfectly dug:

Stanley walked to Zero's hole and watched him measure it with his shovel. The top of his hole was a perfect circle, and the sides were smooth and steep. Not one dirt clod more than necessary had been removed from the earth.

From this passage, we see that digging a perfect hole is one of the only ways for Zero to prove that he's good at something. He can't defend himself and he can't read, but he can dig the fastest and the best hole. Isn't it human nature to enjoy something that we do well?

Then again, it's possible that Zero doesn't really like digging holes at all. At the end of Chapter 12, Mr. Pendanski tells the boys that there's something special about all of them:

"…Even you, Zero. You're not completely worthless."

It's worth taking a close look at Zero's reaction to this. First, he stops smiling. He shuts his mouth tight. He glares at Mr. Pendaski. It's clear from these actions that Zero is not happy. Then he responds with "I like to dig holes."

Should we take that statement at face value and believe that Zero really does like digging holes? It's more likely that Zero is being sarcastic in response to Mr. Pendanski's incredibly insensitive question. 

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