How relaxed (or not) should a sprint be?

Question

What should be the attitude towards getting stories done that are assigned to a sprint? Obviously you want to prioritize getting them done in the sprint, but to me the whole point of agile is to be dynamic: You don't want to deliberately procrastinate or make it "ok" to miss finishing user stories in a sprint, but at the same time when unexpected things come up, and those stories do not get completed and are pushed to the next sprint, you don't want the feeling that you did something wrong. That shouldn't be a scary or negative experience, should it?

Are negative/scary experiences acceptable for missed sprint commitments? Should developers be held accountable for missed sprint commitments when unexpected tasks come up that must be dealt with (Eg. production support)?