I have the uncanny ability to ruin any special moment I am in. It's not exactly the kind of ability that would let me join the X-Men (though I do often get called a mutant). What it comes down to is that I cannot shut off that thinking part of my brain. And I am such a witty comedian that everything somebody says I can (and generally do) turn into a joke or insincere insult.
This can have pretty disastrous consequences. The worst comes when people are being heartfelt. When somebody drops their defenses and exposes themselves, and all I have to say is some inappropriate crack, it cuts deeper than anything else (because they dropped their defenses) and it also makes them never want to be serious with me again.
If somebody is being kind, don't be a jerk. Don't correct somebody's grammar when they are telling you they like you. Don't say "that's what she said" when somebody is telling a traumatic story about childhood abuse.
You'd think this is common sense. And it should be. And most people may not have this problem most of the time. But a great deal of problems occur because somebody just could not shut up for their own good.
Whether this applies to you or your characters, it probably applies. Think about it.