Passion about what you do is what will keep you motivated to do better. With passion comes the need to learn more and be updated in the industry you are in. Passion forces you to improve on good methods to make it better. Passion makes you want to do the best that you can do.
Sorry to be the porcupine in the field, but I have some affiliate products I promote that I have very little emotional interest in or passion about. But, they're in high demand right now so they are definitely in my "financial" interest! .
You could have all the passion in the world but that's not going to get people to buy if they're not already spending in that market. Shore is a lot safer to find what people are spending their money on and become "passionate" about getting your product in front of them.
Just wanted to point out that having passion and interest isn't the only way to get a business going.