this kind of stuff in iOS drives me absolutely batty. like the "omw" -> "On my way!" substitution that it just seems dead set on throwing in all the time-- who on earth thought of that shit?
To be an armchair theorist with only anecdotes real quick, I could see this phenomena maybe being a way of adjusting to communication without body language; choosing between a period or not being a rough equivalent to relaxing with a friend or having a serious discussion. This is at least how I see it in most groups but there are enough exceptions it wouldn't be fair to merely call them exceptions.