As with most things.. it depends! I think Thunder is most accurate, but it can have to do with someone's background, upbringing, things like that. (Steph made a good point too, just caught her post right now) I think some people are just callous and don't really care how their interactions affect others; sometimes, it's a mental issue (sociopathic) or it's just plain self-absorption. We see these people all the time on the road, at the grocery stores.. wherever. It happens and sometimes, it's just best to leave them be, to be unhappy with their lives and not allow them to affect you or yours. They're not worth it in that sense. BUT, if you choose to shed some light on them, perhaps encourage some dialogue with them, they might give you a glimpse as to what they're struggling with and maybe in that way, you can help them. Or just pray for them.
Me, I just do for me and mine. I wasn't raised to be a jerk and I've found that being one, one who always has press the buttons in order to feel like proving something isn't the way to be. If someone tells you something in confidence and you blow it, you look unreliable, untrustworthy. All you do is look like an idiot who has no life.
I try to respect people's boundaries, in real life and online. I try my best and if I've ever crossed a line that really wasn't beneficial to cross, I would hope someone's called me out on it. So far, I think I'm okay with how I conduct myself. I try to respect that people have lives of their own and have their own privacy. The only time I think I'd cross a line is if I feel as though I hadn't spent enough quality time with someone.. I dunno, I'm trying to think of an example lol.