ICMP. It's like "Whazzup"... and they're like "Dude!".
Occasionally you get people that aren't listening or decide not to answer back because they are better than you or blocked by the router.
I'm from the South, and here "Whazzup" is translated to "How's it going?". The response is usually "How's it going". Yep, ping-back. Rarely do we respond with any useful status, making it much like ICMP.
If you did reply back, it's expected you reply back with "fine" whether or not you are fine or not, since no one actually wants to hear about your problems. Imagine if they actually asked for more information? That's like responding to a ping with a full nmap port scan. Not cool.