When does teasing becoming bullying?

Personally, I think teasing can become bullying if it goes on for too long, becomes too personal, is intended to amuse others at another person's expense, and is sincerely not meant by its originator to be harmless. Taking low blows at people based on their physical appearance or anything else they might be highly self-conscious about is not simply teasing; it has crossed the line at that point.

Granted, some people are overly sensitive and do have a shit fit even when someone makes a remark that was clearly intended to be a harmless and playful jest, or an honest attempt at giving constructive criticism. I do acknowledge that this happens, as I have met such overly sensitive people and they annoy me quite a bit because they are difficult to talk to because you always feel you are walking on eggshells around them. But if you know someone is highly self-conscious about a certain thing, and you continue to get on their case about it, then you are not doing their self-esteem any favors and this can easily result in bullying. If you act differently towards someone based on arbitrary or superficial aspects of their appearance or little behavorial quirks, you are bullying in a sense. Going out of your way to ignore someone, especially when they speak to you directly, can be every bit as mean in its own way as giving them too much of the wrong kind of attention. Neglect from your peer group can be every bit as hurtful as neglect from your family. We need to feel that a good proportion of the people in our lives respect us and value our presence, and if too many people we spend a lot of time with are constantly saying things or behaving in such a way that makes us feel like we are not respected and not valued, or making comments that diminish our self-esteem and are deliberately calculated for that purpose, then it is indeed a form of abuse. I used to have a very valued friend who had a tendency to tease people too much and too relentlessly, and he was fond of taking really low blows and deeply personal jabs at people in front of others (for instance, he would continuously call a friend of his who lost a testicle in a car accident "One Nut" in front of me and other people who were not privy to the accident prior to his remarks). He would frequently tease me in front of other people about aspects of my physical appearance that he knew I was very self-conscious about, and when it finally went beyond tolerance after an hour of it and I would sternly tell him to shut the fuck up, he would get perplexed and say, "Can't you take jokes? I was only kidding!"

When people who are categorically not your friends behave this way towards you on a regular basis, and encourage others in your peer group to join in with them, it clearly goes beyond simple teasing and becomes a form of abusive bullying.