Why did Germany declare unrestricted submarine warfare two sparate times?

Problems in restricted sinkings: 1. Takes too long to make the kill. 2. Submarine has to stay in one location & surfaced in one location too long. 3. Survivors abandoning vessel to be sunk present a problem; life-boats, food, sick and injured, etc. 4. Extended time on station (see #1) allows allies more time to locate, target, and destroy submarine. Benefits of UN-Restricted sinkings: 1. Saves submarine and crew from attack and being sunk. 2. Allows higher kills at sea, at minimal threat to submarine and crews.