When it comes to bullying in the workplace, or any other place come to that, unless you know your vulnerabilities, you ARE vulnerable.
Some targets, almost on principle, assume that just because they are ‘nice’ and reasonable, then everyone will be reasonable with them.
Unfortunately, not everyone abides by the rules of the reasonable.
For example, was there something you weren’t very good at, at school – did someone exploit that to make you feel bad?
Then the same could happen at work. A workplace bully will test that boundary by ‘gentle’ teasing; how much of that would you take before calling time on it?
You need to be clear what your boundaries are, in any given situation, and what you will say and do should someone test them.