Software Testers focus on Software Users

Knowing what the user wants is important in producing software.
Also, knowing how the user would use the software is important to producing quality software.

Software testers should focus on both: understanding user requirements as well as how an user might interact with and use the software. This understanding helps testers test scenarios that are closer to reality.

It is usually straight-forward to come up with the positive tests. Even developers can do it to a large extent ! When developers develop software they have an expectation of how users "should" use and interact with the product which is related to how they have designed the system.

Smart testers design their tests, especially their negative & error tests keeping in mind how users would behave. In the real world, users will make mistakes as part of the learning process, not read the complete documentation or manuals for your product, interact with the product in ways the developers do not expect, provide inputs that may not be the values that your system expects and do various other things that could show up chinks in your product's armor. In fact, we all make "mistakes" as users of different software products. As we try to familiarize ourselves or explore the product features, we end up doing things that the developers of the software may not have envisaged. Testers need to incorporate testing for errors, "nonsensical" actions, invalid inputs, etc. to try and mimic real world actions by users.

This user focus by testers, translates into how defects are reported. Testers assign a severity value to each defect which reflects the tester's estimation of impact of the particular defect on the user. Severity also factors in the likelihood / frequency of users facing the issue. To focus on users, testers need to be encouraged to think independently and not just go with what developers think testers must test. Developers come with a perspective of how the system is designed and their baggage of expectations on how the software should be used. Getting testers exposed to customers/users or customer facing groups could help them to approach testing with the user perspective in mind.