We all make assumptions. It's the only way we can get anything done. If every time you found a bug you started at the iron - testing the CPU to make sure every operation returns an expected result - it'd take you months to troubleshoot the simplest issue. So we make assumptions to save us time, when we know that the likelihood of something being the cause of a problem is far less than the time it would take to verify it.
We also make assumptions out of sheer bloody-mindedness. You can spot these assumptions by phrases like "that couldn't possibly be it" or "it's never been a problem before" or "I wrote that code, I know it works". These are the kinds of assumptions that can get us into trouble, and they're the exact reason why it's important to have developers from different backgrounds, with different perspectives, who make different assumptions.
Since we all make assumptions, the best way to challenge those assumptions is to have someone who makes different assumptions look at the issue. They'll bring their perspective and experience to the matter, challenge your assumptions where they don't make sense, and make you prove those assumptions to be accurate or not. This stands as a strong incentive to hire a team with diverse backgrounds and areas of expertise. They bring not just talent to your team, but a different perspective.
It's also a good reason to invest the time in learning different technologies, languages, and development philosophies. Getting outside of your comfort zone can open your eyes to things you might not otherwise have considered, and help you to gain new perspective on your work - helping you to challenge your own assumptions.