We shouldn't let the devil steal that joy, or worse, just give it up. When we give up our joy, we start to focus on little things, and get our eyes off God. When we take our eyes off God, it is all downhill from there. Though this world is full of trials and persecutions, we should remember that even Jesus was persecuted.
Even when incurable diseases happen, we can still have joy in God, because if that person is saved, they go to be with God. It can be really tough on those left behind, but God won't give us more than we can handle with Him.
Though, at times, we don't understand what is happening, we can be sure that God has a plan and is working things out. We probably won't know the whole plan, but He does.