Because God wants to take you in ever-increasing measures to know Him. And the only way you can know Him is by experiencing Him. So He's going to ask you to go with Him in dimensions that require more faith and more activity than you have ever used before. Otherwise you will never grow in your faith in Him. The only way you can grow in your faith in Him is to accept the next assignment which is always greater than the previous one. Don't ever feel that you will get to the place where you'll never be scared half to death.