I think the reader's point of view contributes a lot to how he or she interprets a post.
Example: If I am overweight and have body image issues, odds are that I will (most likely) be very touchy about comments about diet/exercise/size etc. I may read into a comment a personal attack on me - because that is where my mind is.
If I have been criticised - or if I have insecurities about my quilting, then I may be taking a well-meant critique as a personal attack.
Sometimes the choice of words can trigger emotional responses. Example: The comment "if you don't have anything nice to say - you shouldn't say anything at all" sends me to the moon and it takes a lot of self-control for me not to answer. It wouldn't be pretty (or nice) what I would have to say - lol.
We also jump to conclusions at times because we interpret a message in a certain light. Instead of asking "how did you mean that?" we react and answer in a less than constructive manner.
All in all though, considering how many of us there are - I think we are doing pretty good.