When you design or change something, you’re doing it for a reason. They want to know what that reasoning is. It's not necessarily that they don't trust you. As a programmer I might ask why something is being done because I want to know how it impacts the rest of my project. People might generally trust your advice, but there will always be cases where you need to explain your opinion no matter how far along you are.
There are many different standards which you may be relying on when you make a design decision. You aren't going to follow standards all the time, but if you break away from the norm you should have a reason for it. Common standards and examples:
Platform specific UX standards:
- Color theory
- Focal Points
- Visual Grouping
Company Specific UX Design Standards
- Usually laid out to give things a consistent feel
- Color palettes, specific fonts
- Based on studying how users react to things.
- User eye tracking
- How people generally expect things to work, because that's how it worked everywhere else.
- Example sites: www.nngroup.com , www.usability.gov
- User tests
- Is it acceptable practice to use checkboxes as radio buttons?
The resounding answer with 27 upvotes was "No". Because it breaks the standard of how that control is usually used. And it makes sense, after all if something suddenly works differently than most users expect, it will probably throw them off. You wouldn't want to flush the toilet and have the lights turn off.
(Explaining visual design principal)
Q: Why are we moving this button to the left?
A: Items that are grouped together are usually seen as one item. We
wanted to more strongly associate button1 with button2.
(Explaining how something is usually used; user expectations)
Q: Why are these circles? I thought I said I wanted checkboxes.
A: It’s standard to use radio buttons when you are only allowing the
user to select one option. It conveys the fact that they can only
select one without having to tell them that. This saves time, makes
the sign up process faster and thusly raises customer retention rate.
Q: Oh? What standard is that based on?
A: You can read about it here if you'd like to know more. http://www.nngroup.com/articles/checkboxes-vs-radio-buttons/
(Color theory, flow)
Q: Why did you change this to light blue?
A: To contrast against this dark blue, which makes the flow of the
website more linear. This strains the user's eyes less and allows them to read more content.
(Broadening the questioner's scope of project)
Q: Whoa! You can’t make the lights turn off when you flush the toilet!
A: I know that isn’t something that is normally done, but even though
the light over the toilet turns off, the lights over the mirror turn
on first so you’re not sitting in the dark.