You actually do not need to use shampoo. In fact, it's probably healthier for you to quit using it completely.
In a blog post, Nat Eliason describes his journey of not using any shampoo for years. I'm not going to repeat the arguments listed throughout the blog post, so you should check it out yourself. Inspired by this concept, I took on the challenge myself and started on 2021/06/28.
While my hair didn't feel as smooth initially, I can call this experiment a success after trying it for about a month and overcoming the adaptation period. The length of the initial adaption period is different from person to person, so be prepared for it to take up to a month.
My hair got more voluminous and looks better overall. Additionally, it also stays in that shape for longer! Back when I was using shampoo, I had to wash my hair on a daily basis to maintain its look.
All in all, I'm convinced that using no shampoo at all and only rinsing your hair using water is actually good for you. But keep in mind that you have to live a healthy lifestyle in order for this to work. Without shampoo, your hair is only as healthy as you are.
Finally, there seem to be other cases in which less is more: Some describe quitting face wash as being healthy for your skin. Instead of repeatedly damaging our skin's natural moisture barrier, you keep it intact by only washing your face using water.