"…and here’s why…"
"...and here's why..." might be the three most important words for you as a designer, writer, or product manager, and here's why: Everyone thinks they can do your job.
(They don't mean to be malicious about it, but think of how many times you've heard: I think it would look better right-aligned" or "Can we say 'internal' instead of 'private'? or "I'm smart and I feel like I have good 'product instincts'...)
And unless you are vigilant about using"...and here's why..." in daily practice, they'll continue to think that way.
You're a designer.
When you make a decision on line-height or color or how much information to reveal in one interaction, you're basing that on a combination of technical design skills, knowledge of human psychology and visual perception.
You're a writer.
When you make a decision on sentence length or using specific words or choosing a friendly/serious/casual tone, you're basing that on knowing not just the meanings of words but subtle connotations and understanding reading (or let's be honest: skimming) behaviors.
You're a product manager.
When you make a decision on which feature to cut, what metrics to track, or when to ignore customers who say one thing but do another, you're basing that on market knowledge and understanding tradeoffs between time, features, and quality.
And of course, the experience of having seen what has worked (and not worked) in analogous situations in the past.
But to your average co-worker, that foundation is invisible.
They just know "I like it" or "I don't like it" or "here is my personal preference."
This turns what should have been a conversation based on experience and objectives into a debate based on personal opinions and who can speak louder (or persist for longer).
This will happen unless you own your recommendations:"we should do X, and here's why..."
Then the conversation can remain elevated. People may not agree with you, but now you can clearly drive the conversation. Now the other person can clarify that they disagree with your "why", or how you chose to solve the "why". You can figure out whether the issue stems from a miscommunication, poorly-stated (or changed) objectives, or the other person having knowledge that wasn't shared with you.