One of the reasons it’s useful to engage with people who don’t agree with you is that it forces you to think much more rigorously.
It’s easy to get away with sloppy thinking and careless arguments when you’re preaching to the choir. As long as what you say supports their beliefs, they won’t point out the logical inconsistencies and hidden assumptions.
Why keep putting in the work to test your arguments and find supporting evidence, when all it takes to get a positive reaction from your audience is to tell them what they want to hear?
The great Mark Twain wrote a classic story called “The Man Who Corrupted Hadleyburg.” In the story, the town of Hadleyburg is known for being incorruptible, and the residents are carefully trained to avoid temptation, as evinced by the town motto, “Lead us not into temptation.” Naturally, this makes them vulnerable to the titular man, who reveals that the residents simply haven’t had the opportunity to behave corruptly, and blows the lid off their hypocrisy. The town realizes the error it its approach, and changes its motto to “Lead us into temptation.”
When you limit yourself to interacting with people that agree with you, you are like the residents of Hadleyburg. Maybe your arguments are strong and persuasive, but how would you ever know?
Only by interacting with those that disagree can you test the mettle of your beliefs, and your ability to stand up for them. The goal isn’t to win over your audience; it’s to hone your own thinking to the sharpest, most precise, and effective edge.