Dehydration can cause headaches because it leads to a decrease in blood volume. This can cause the blood vessels in the brain to constrict, leading to headaches. Dehydration can lead to headaches in some people by causing the blood vessels in the brain to stretch.
However, there may be other reasons dehydration causes headaches, and not everyone is equally susceptible to this type of headache. Increasing water intake may help reduce migraine symptoms in people who experience them.