What's more important, using agile practices or understanding agile principles and values? Both! But what if you had to pick? Consider this:
If you're happy, you smile. But did you know that, if you smile, you become happy? Somehow the act of smiling connects your brain to a feeling of happiness.
This is interesting to me because, like anybody, I get grumpy. If somebody saw that I was down, she might say, "Cheer up!" or, "Don't worry, be happy!" (Annoying, isn't it?) I already know I need to be happy. Sometimes I just can't seem to make it happen.
That's what makes the smile research so interesting. If somebody told me, "Try smiling for ten seconds... it seems silly, but it will make you feel better!," I might actually try it. Unlike, "be happy," it's something I can actually do. And, as it turns out, smiling actually does make you happier.
Practices or principles? I don't know. The principles are important and valuable. But if I could only have one, I think I'd choose conscientious and careful application of the practices, in hopes that the act of smiling would help me rediscover my good mood.