I think, at least based on my mileage, that it's usually due to past experiences that maybe went poorly and people are trying to learn from. For instance over time I've developed a loose list of requirements/"nice to haves" for people I'm dating. They're relatively lax (some of them will make you say 'no duh') but still, they've developed over time and I'll probably only add more as my experience grows. Stuff like: - should be in my generation (i.e., if I am in my 30s, should be in my 30s - the idea is to date someone close to my age) - Must have never gone to jail - No kids - No divorce (Disclaimer; I am not in my 30s; if someone at my age is divorced I'm going to consider it a red flag) Stuff like that. It's not nearly as specific as hair color or pants size.People care about the oddest things.