Do you have zero interest in an expensive mountain bike the company you are an affiliate of sells? Well, you probably don’t want to feature it on your blog, as it is extremely difficult to persuade readers (or anyone for that matter) that they should buy something you wouldn’t be caught spending a single penny on. When you are passionate about a product or–at the very least–interested in learning more about it, this will come through to your readers, engage them and better coax them to buy
Another consideration to take into account is price point – what is your audience willing to pay? Believe me; you're unlikely to make sales promoting products and services that are outside of your audience’s money comfort zone. Honestly, it took me a long time to narrow down who my target audience was, but if you’re focused and take the time to do the research, it shouldn’t take you that long to figure it out.
At the time, I had a ton of people reaching out wanting to hire me (I ranked my self #1 in Google for WordPress SEO Consultant, WordPress SEO Expert, many other good keywords). Unfortunately I struggled with basic things you need to run a service-based business… keeping track of clients, time management, and making sure I was charging clients for my time (and getting them to create content which often seemed impossible).