Why do you think Daniel took a stand concerning being defiled by the king's food and wine? How do you think eating the king's food would cause defilement to Daniel's conscience? What does this tell you about Daniel?
Here we see 4 boys, led by one in particular, and who knows out of how many total, that was probably just fed up and tired with all of the changes (and frankly, possibly some of the nonsense that was being presented to him as factual). There are times in life when we are in situations or dealing with personalities that over time may seem wearing on the soul - it's tiring to have that feeling that you are pushing against a wall in your attempt to move forward (in life). And that may have been what drove Daniel to take a stand - he can learn the language, he will listen to their thoughts on educational matters and theory, but now you are just crossing a line with telling me what I need to put inside of my body...especially considering that the food you want to give me has been food I have grown to detest even the site of. And he took a stand.
But even if my thought is correct that he was tired and fed up with all of the changes being forced up on him, he doesn't allow that to show through in his conversation with the official as he presents his request for an exception. He remains calm and controlled and while we readers today do not discover until the new testament that we are more than conquerors through Christ, I think that Daniel may have already had a concept of God's favor on His people and those who seek to follow Him wholeheartedly and how that applied to him personally. Oftentimes we are able to be more bold or ask for things that others may be to shy to (as we see here) because we have an understanding that we are a unique and purposeful creation. Having this level of comfort and confidence allows us to maintain a peaceful nature when approaching or in the midst of a conflict.
Daniel may not had needed to go to the extreme of eating only vegetables and drinking only water but what I love about the outcome here was that when the time came for a comparison to the others, there was no room for the king's food / program to receive the credit - once again we see that Daniel, knowingly or unknowingly at the time, sets up for God to be the only one that could receive the credit for the success and continued thriving of these boys that had been set apart. With his actions in refusing to eat the king's food and masterful navigation for this be an acceptable exception for not only himself but his close friends, Daniel shows a maturity in his knowing of who he was, what he believed and where (Who) he came from that I think many individuals never reach.