For product, \(X\), in the universe of products, most of the time:
But, the part people seem to forget is,
Verbose Version 
"Scratch your own itch." Prescribed as a heuristic for entrepreneurs searching for a product, it means build something that you yourself need. It can be a good heuristic. You're not that special -- if you have identified something you need, it's reasonable to assume other people have a similar need. Except, it's only a good heuristic for building a business if you are good at assessing your product's value. This is challenging if you have not learned to temper the instinct to fantasize.
Assume that you have just identified a product that you want, in the abstract sense. That is, you find it attractive at the moment of conception. Propelled by the desire to create, to build, and to be rewarded, you draw increasingly elaborate visions for the success of your product. You plot a fantastical path between now and your incipient empire. The future is rosy.
Then, with palpable trepidation, you do a cursory search to see if anyone else has had your brilliant idea. They have; they've already built it; and, it's better than what you envisioned. Their product scratches your itch perfectly and gives you a back-rub as well.
...and, you don't buy it...
Partially, your sense of value may be momentarily skewed due to a form of anchoring bias. You were thinking that this product would mean a billion dollar valuation. With that fantasy and its concomitant hopes deflated, it's difficult to maintain rationality. In the mathematical sense, the benefit of their product may exceed the cost -- but your utilitarian calculus isn't operating correctly. Any benefits seem insignificant now, not worth the time to acquire. A poor man picks pennies up from the ground; a rich man does not, and moments ago, you fancied yourself a soon-to-be-rich man.
Still, ignoring the anchoring effects, there is a more obvious (and probable) explanation for your failure to "scratch your itch" by buying their product -- the product isn't actually valuable to you. It's not even that valuable to anyone else. It's just a novelty that became a vehicle for fantasy. So, while "scratching your itch" may be a good course heading when navigating the space of possible products, it is not magical. You still need to Think Hard and engage in skepticism. There are many more unviable products than viable ones, even after filtering for those that you (seemingly) need.
 The ideas behind this post percolated while reading Zero to One: Notes on Startups, or How to Build the Future,Peter Thiel's new, (rightfully) much-lauded book. Some credit indirectly belongs to him; any blame and misunderstandings, to me.
 Thanks to @RaananKagan for providing feedback on the first pass of this post, prior to publication. Also, thanks to @DannyHorowitz for feedback on the second, and for pointing out the value of iteration and testing. Although I didn't want to include it in the main body of this post for fear of diluting my main argument, if you are continuously testing your idea against customers, you're essentially hill-climbing the product space. This practice can be yet another tool for squeezing out the unviable in favor of the viable.