A. Some sharks are very desirable hosts for small parasitic creatures. Those sharks are constantly trying to escape from them by incessant swimming.
B. Sharks are highly anxious and easily agitated animals. They cannot stay in the same place for more than a few seconds.
C. Sharks are cold-blooded animals. If they stop moving their body temperature will decrease too much and will cause hypothermia, which can be lethal.
D. Some sharks have lost the ability to pump water through their gills, so they need to keep moving in order to stay alive