The point is that NPCs often say things that either don't matter and bear no consequence in the grand scheme of the game. Worse, they're often just dull comments that lack any real character to them. And they just get blurted out. Blurt is probably the biggest sign that something said is indicative of NPC behavior. I don't remember talking to that NPC, so why does he keep saying the yellow flowers are his favorite whenever I walk near the garden?

Mentioning the weather is never a good idea - someone two feet away from you doesn't need you to say "it's cold" when they are literally feeling the exact same thing you are. What's worse is if you say the same thing often, especially to the same people. Imagine how one must feel to hear you complain about the humidity unprompted for the ninth time in a week. They must think you are an NPC.

Of course, I'm being mostly self-critical here. I do this. And yes, I need to stop. Realizing it is the first step - the second step is to work on correcting the mistake.