I empathize with Peter. He was probably trying to snap Jesus out of what he thought were depressed thoughts. He wanted to comfort him. But his simple words became a greater test for Jesus. It could have implanted the thought in Jesus' mind that if God loved him then he wouldn't have to suffer.... Jesus knew better though.

We do the same thing to people today. In trying to comfort them we sometimes tell them that God will not allow bad things to happen. But in this world bad things happen everyday. People die, spouses leave, children get into trouble... God does not prevent things from happening just b/c we may see them as bad. He allows tests to come into our lives to help us grow, He allows trials to make us stronger, and He even chastizes us when we've screwed up.

In light of that I think we need to be careful in what we say to comfort other people. Maybe a better expression would be from Romans 8:28.

Matthew 16:22-26