1. Shampoo is expensive.
2. Chemicals in shampoo can irritate and dry out your scalp and hair.
3. Chemicals in shampoo can be hazardous to your health.
4. Your body produces oil to naturally condition your hair and keep it healthy. Using shampoo daily upsets the natural balance and causes you to produce more oil than necessary to compensate.
When you stop using shampoo, you will probably experience a transitional period while your body gradually adjusts to produce just enough oil to keep your hair healthy. During this time, which can range from a few days to a few months, you'll probably have some extra oil and grease that you may want to take care of to look presentable. The cheapest and most effective way to handle this is to use baking soda. The method I use involves a plastic cup, some baking soda, and a shower. You may also want to condition your hair after using the baking soda mixture, as sometimes it can leave your hair a bit dry. There are lots of different ways to do this; I use apple cider vinegar.