One reason that critics oppose missile defense systems, such as Reagan's proposed Strategic Defense Initiative, is that they view them as undermining one of the fundamental premises of mutual assured destruction: The proposed defense systems, intended to lessen the risk of devastating nuclear war, would in fact lead to it, according to critics.
The non-missle defense side, seeing that a nation was building a defense against a first strike and believing that the other could launch a first strike if it dared, would then launch a pre-emptive first strike while they still had a chance. The reasoning behind this is the claim that mutual destruction is better than defeat.