Other good reasons to learn to say no is for your own health, peace of mind and familial/social relations. I have seen many solopreneurs working themselves to death out of fear of saying no to clients, and it truly affects not only their personal life, but the level of service they are able to give their customers. One of the most crucial things taught in first aid is relevant here: It is essential to make sure that you are ready both physically and mentally to take care of people, your business, before you start taking care of people. You cannot serve your clients well if you do not keep time for yourself and your loved ones.

Besides that reason and the reasons in the article above, most of us got into business for ourselves not just to make money, but to take control of our own lives and our own destinies. If you are saying yes to every client, even the ones you do particularly not want or cannot serve well, you're not really working for yourself, you're just working and reacting for other people.