Entrepreneurs are inherently optimistic—hard-wired to see the opportunity in everything—but a constant view of your business through rose-colored glasses is dangerous. You need to ask yourself hard questions about who is going to pay for what you're offering and why; why you in particular are suited to start and run this business; what skill set your business will require that you suck at (and how you'll make up for this); and whether your resources (money, time, expertise) and motivation will be enough to sustain you through the inevitable challenges. If you can honestly answer these questions and feel comfortable with the results, than you are probably in a good position to start your business.