Sometimes I will enter discussion wanting to talk about X, but my correspondent seems keen to jump around from topic to topic. They will respond with a question loosely about X, and I feel obliged to respond. We're now talking about Y. They will then ask a question loosely about Y, and I feel obliged to respond. We're now talking about Z.

Before I know it, we're talking about L. I don't want to be talking about L. Fuck L. I wanted to be talking about X. I probably would have been content to talk about Y or Z for a while if we could stay on them for any length of time, but we can't, because there are so many other letters and the person I'm talking to can't resist them. I've learned to nip this process in the bud. Sometimes that's with a polite "I'm sorry, but I'm not interested in discussing this". Sometimes it's by ignoring them.

TL;DR: if I feel my correspondent is going to take the discussion on an unproductive wild goose chase, I may stop responding.

ETA: I'm not necessarily blaming my correspondent for being a bad discussion partner here. It may very well be down to different expectations from the discussion. I've probably come across as an attention-deficient topic-hopper myself on more than one occasion.