'For God to suffer would mean that God is a part of the created order. For God to suffer would mean that an aspect of the created order (not to say that evil, which causes suffering, is a created thing – it simply ‘exists’ in the created order of nature) could affect the uncreated order. Thus God’s wholly other-ness and transcendence is violated, making Him part of the same created order within which evil works. God then becomes little more than a being-among-other-beings.'
Another blog post on divine impassibility.