Have you heard the term niche website or niche marketing? This has to do with breaking your general site idea down to a much more targeted market. But there does come a time that you can make that niche too small.
First of all you need to research the keyword idea. Some obscure niche’s can bring you money. If there are 3 people a month looking for aluminum dog boxes and there are only 10 sites specializing in them. You may have a market if those dog boxes sell for a high markup. However if you are selling handpainted mailboxes of flamingos and 3 people search for them a month, there are 3000+ sites to choose from and you barely make a few cents on the product, then it is a niche that is not a smart choice.
Choosing a profitable niche is of course a major decision and can make or break a site. But, there are many ways to tweak that niche down the road. So do not trash the site if it does not make money immediately.