It is inconceivable that one claims to design user experiences, focus on user-centred design, or apply heuristic principles to products without conducting regular and thorough user testing. Period.
One can't be pretentious to think one's assumptions and decisions work for others the way one thinks they would. More often than not, they won't.
We live in our own context. In our own bubble. This bubble is determined by our environment, the people we interact with, our preferences, and interests.
It goes without saying that unless you design something for yourself or for people within the same context as yours, you are not in a position to presume your assumptions are valid. If you do so, the odds are significantly against you.
This is why user testing is so seriously fundamental.
You know, we are keen to ask why. It is rooted in us. Part of this "ask why" mentality relates to pushing the limits. To create something better. Something different. Every time. Not for the artistic urge to be different, but because there are invariably more, better ways to solve a problem.
It is analogue to science. Science is constantly trying to prove itself wrong. Not right. Wrong! New hypotheses and experiments are continually held to forfeit the status quo and, as a result, push knowledge forward. You don't see a scientist going: "I believe the cure for AIDS is bleach pills! Let's get that going, shall we?". How mental would that be?
We must face Interface Design likewise. Observe through user and competitor research. Question how we can improve from there. Hypothesise new ways of solving that problem. Design it. Test it to prove yourself wrong until you are right.
Even then, you will be wrong. But right-er.
Testing and being proven wrong is not a mistake. Is to take a step forward towards something better.