"Complexity is not just a necessary evil and not just instrumentally good. Given the right circumstances, encounters with the very complexity of some systems and behaviors can give us fullness, entertainment, aesthetic and sublime experiences, spur and develop our abilities and ambitions, and give depth to our experiences and understandings [Nelson 2007; Csikszentmihalyi 1990].
Rather than being a universal human ideal, simplicity is often disapproved and derided in our everyday lives. Being simple can provoke condescension and even contempt. On the other hand, humans seem to seek and enjoy certain experiences of complexity. Sometimes complexity may be understood as richness, generally found to be a positive and wanted quality. The experience of being in a forest with its overwhelming profusion of different life forms and natural structures is seen as richer than being in the controlled and simplified park. The simpler an environment is, the easier it is to understand and handle, but at the same time it lacks the richness and stimulation that we appreciate and enjoy. (Gardner et al.  discuss similar aspects when examining what constitutes “good work.”) Many natural phenomena, such as a river or the weather, are in our daily lives not experienced as problematically complex, but when we want to control them or understand them in a scientific sense we need extremely complex tools, systems, and explanations. These natural phenomena are of course extremely complex, even though humans live with them without feeling intimidated by their complexity.
There is something intriguing about complexity. It constitutes a challenge, something we can explore and experience, something we can attempt to learn and attempt to master, and something that we know can send us off into new and unpredicted directions, something that promises adventure.We can gladly spend a lot of time and energy trying to figure out the mechanisms behind a complex behavior or system. We can even be prepared to devote our life to the complexity of something such as wine or music."
If our reasoning is correct then complexity is not by default bad, or at least not always something we should try to simplify, reduce or remove. We have to be careful when dealing with systems that people are engaged with. Reducing complexity may lead to reduced richness which in turn may lead to poor and simplistic human experiences with those systems. It is of course true that some systems may be better with less complexity, sometimes simplicity is truly what is needed. But when is complexity good and when is it bad? And how is it possible to know the difference?
First of all, there is no guarantor when it comes to this kind of questions. There are no 'correct' answers. Designing a complex interface may be a solution to a safety issue, while the same design may be cumbersome when the task at hand in a everyday repetitive 'safe' task. The challenge is not to be able to make everything simple, the challenge is to know what is an appropriate level of complexity given a particular task and situation. And then, of course, to be able to design that level accordingly.
This all leads to the notion of design judgment. There is no other guarantor in design than the judgment of the designer. There is nothing 'out there' that can tell a designer what the 'right' design is. There are no correct solutions. There is no design principle, approach, method, or tool that will deliver the 'best' design. There are no patterns or templates that can reduce the risk of designing. There is only the ability of designers to make good judgements.