People in other professions have similar issues. Some 'smart' folks feel as if they should be able to understand everything. It probably often works for them, after all, how did they become 'smart'? But, it's a fine line between genuine interest and distrust per se. Try to assume good intent, consider it interest and grow your ability to explain. At some point you might have to call it as a poor use of time and move on, but if you can use the time to grow on both sides of the discussion - bonus.