One of the foremost rules of real estate and property development is as follows: Develop for the market and not for yourself. To take an extreme example, you might really love Justin Bieber. You might think “I’d definitely buy a house that had a huge picture of Justin Bieber on the wall, one that was impossible to remove or paint over!”. But you’re selling the house, not buying it, so what you like doesn’t matter.
That’s not meant to sound callous, simply to illustrate a point about how you make money. What you think is worth buying is not the important aspect when you’re trying to sell. To maximize your chances of selling something, you have to give it mass appeal. Too many people get emotionally attached to developing a house for sale and make it something they’d buy, without thinking about the market.
The same goes for monetize a website. You might really like a particular niche – it can have a particular emotional attachment for you and you will find it easy to write enthusiastically about. Placing ads on that kind of content might well attract a small number of clicks from people who are equally enthusiastic about the subject – but it will not bring in much in the way of traffic.
This does not mean that you should refrain from setting up a website about a subject that enthralls you. Just don’t expect it to be your money maker, and if you want to make big money you should look to develop a site which will have mass appeal and will attract clicks. Any money you make from the more obscure niche that you love is a bonus, because you like the subject matter anyway.