I would definitely have to agree. If you keep in mind that this is your business for the long term, you want to choose something you aren't going to be sick of after a short while.

You don't necessarily have to know a lot about a topic, although it is easier that way. But as long as you don't mind the research, you can build a site about almost anything.
It is a lot easier to dominate a niche in a shorter amount of time when there is less competition. If you choose an extremely competitive niche (like network marketing or auto insurance

) be prepared to put in a LOT of time and effort before you get the results.
For example, my insurance site is 7 months old and my parasite site is not even 3 months old. The parasite site gets better rankings and traffic already than the insurance site and if I continue to add content at the present rate, it will overtake my chocolate site in no time.
There are always those variables to consider, but a narrower niche is usually going to bring you faster results - which is a good thing for your first site - to keep you encouraged.

If you are talking about non-website affiliate marketing, the answer may be slightly different. However, you will still need to come across as confident and knowledgeable in your writing if you expect people to take your recommendations and click on your affiliate links.
When you know and love a subject/product, that becomes much easier to do.
Blessings,
Angie