Something related to a life-long hobby or interest is ideal. It is possible for a business to be generating excellent sales and profits, and yet be losing cash. You need cash to pay bills. A cash strapped business is a business that will struggle to grow. Do a cash flow projection for your start-up business.
Many entrepreneurs start a business and are happy when they see it making money.
They are often not as happy when they realize that profits have plateaued. Think about how you will grow. If you start a restaurant, for example, your profits will certainly level off. You will only have so many customers eating at your restaurant in any given year. To grow would require adding a second restaurant.