I think it's one of the most loving things to say to your fellow best friend. It's a warm and endearing phrase. "Buddy" is nice, but "bro" just has a loving power to it even more. It's much better than some of the rap slang that I've known to be honest, like the N-word.
Bro is short for brother of course, and being that we are all relatives in an objective sense, it makes since to strengthen that fact through friendliness comraderie, rather then focus on our differences. Any dude can be a bro. White, asian, tall, short, college educated, mcds employee etc... COME TOGETHER BROS!
People that use slang think that they are cool because some people can't understand what they are saying. Teenagers nowadays are into video games and going out with friends and that's where they pick up certain words. Why has proper English and grammar been invented if no one is going to speak normally. How can you deal with listening to your older brother or sister on the phone and they are talking in a whole different language basically.
ONE person in the whole wide world can call me "bro" without offending me: my sister. I hate being called "bro" by strangers on the streets and buses because so many of them are criminals and druggies; that's the only real reason they talk so much slang. Earlier this year I discovered that "dudebro" is actually a vague insult.
I don't use the term or find it particularly cool, but there's nothing wrong with the term itself. Is it not a short form of the word brother? This is a term that has been used for a long time, and the shortened version for a number of years. It's not proper English, but conversational English is colorful and contains a large number of non-standard words and phrases. Language is dynamic, vocabularies change as each generation adds their contribution. It is important that everyone be capable of speaking proper English, but there's a place for appropriate slang also.The young, in particular teen and young adults, always seek to create their own popular words. The same is true for other groups-nothing wrong with that Bro!