Means I love you in dinosaur. :D

It's simply quaint way of either grabbing someone's attention (by coppying a creature that makes that sound) or being a combination of cute and aggressive (an oxymoron if you will)

Sorry, this seriously needs to be corrected first.

What does "Rawr" actually mean?

One day, a friend who I hadn't seen for a long time messaged me and wrote "rawr". I thought he was just acting cute.  Later on, I understood it means "I love you" in dinosaur.

Still, I don't get it. Under what conditions do people say this? I mean, whether it is normal for people to use it to say hi.

The joke is, <em>"Rawr" means "I love you" in Dinosaur</em>. This is a cute little internet joke (now a little out of fashion), and you'd only use it if you knew the other person would understand the joke. So, I understand your confusion.  It's not a normal way of greeting people at all, so forget about using it to greet friends. (You will just look weird. Very, very weird.)

Aside from that, I don't know why your friend would write <em>rawr</em> to you. Was he being cute and affectionate?

http://www.urbandictionary.com/define.php?term=Rawr

It can have different meanings depending on the context, but since one of the meanings is this: <em>"more sexually oriented version of the word "roar"" or "a sexy sort of growling sound", </em>I wouldn't say it's a popular way to say hi. Still, for some the definition is: <em>"A noise many people make to start conversations, tell someone that they're bored, and sometimes to say "I love you</em><em>"" </em>

So maybe this particular friend wanted to talk to you but didn't know what else to say to start a conversation. Or they love you.

IT would be nice to know how do they pronounce it (compared to 'roar')