What's with all these hi's

There's another item of etiquette that I find hard to comprehend. Who decided that ALL emails must start with "Hi". Proper and appropriate use is fine and I have no issue with that. In fact, I rather prefer this to "Dear sir".

What I don't understand is scenarios like the following:

I send an email to a work mate who's seated 5 metres away from me. "Hi, ...". He replies. "Hi, ...". I respond. "Hi, ...". He replies. "Hi, ....". How many times are we going to greet each other? Absolutely ludicrous! Unless, that is, the "Hi" in this context doesn't have any meaning and is just the way an email should start. If that's the case, shouldn't this be automated so that people don't have to type it there all the time - it's automatically inserted when the email is sent.

I have always found it extremelly difficult to do things I don't understand, and that's why I refrain from behaviour such as I've described. Some people probably interpret it as me being rude. What's new. I don't like conforming anyway. As I tell anyone who's willing to listen,

Normal is boring