How many times has this happened to you in the context of a face-to-face conversation?
"Sure was a great [insert sport here] game last night!"
"Sure was... Oh hey! I just met my long lost twin bro-"
"SILENCE! I initiated a topic concerning the events of last night's riveting sporting event! HOW DARE you stray..."
Off-topic. Yeah. I'm gonna guess you've encountered this problem precisely...
That's because real conversations - and oftentimes the best ones - are organic. The kinds you have with friends and family. Or even a "good" argument. They don't fit neatly and exactly and exclusively into the little suffocating, ill-conceived box that is "remaining on-topic." At all costs. At the cost of potentially being interesting or learning something new or being free to introduce humor or accomplishing anything that lies outside the established parameters. To say nothing of the arbitrary and inconsistent nature of determining whether or not something is on-topic in the first place.
Because we don't have individual personalities. We aren't varied. We don't have minds of our own. We are merely automatons who ache desperately for guidance and limitation and sweet, sweet suppression. We'd be hopelessly lost outside the confines of the expected and the solicited; weary, forgotten stragglers in the corrupt wilderness that is "off-topic."
Chills, down my spine, go running.