The space shuttle is (was?) a test vehicle. Do you know how many test pilots die each year? Actually I don't know either, but I know it's a lot higher than the number of astronauts.

Before anyone jumps on me, I'm absolutely not saying those lives are not valuable. Of course they are, I regret their loss, and I wish that those accidents did not happen. But pushing the envelope is a risky business, and every astronaut knows the odds but gets in the capsule/cockpit anyway. Because if we are not losing lives now and then for reasons unknowable at that time, then we are no longer pushing the envelope and that would be cause for concern.