So… I don’t think many people really like being emotionally vulnerable, but:

Dominant Fi is kind of weird in that (as far as I can tell) it’s the only introverted function that links up your primary thought process to an involuntary physical reaction.  People are generally pretty cautious about sharing whatever’s going on with their dominant introverted function so being outed by your body every time you’re upset is not fun. 

I want those of you who aren’t Fi-doms to imagine that every time your dominant Ni or Si or Ti points out when someone’s wrong or lying, for example, you cry – no emotional attachment, just your eyes leaking. 

Pretend enough people around you do this that it’s not something to be ashamed about, but it’s considered impolite (like accusing people of wrongdoing without evidence or pointing out the ignorance of someone with no education) and somewhat worrying to do it very often (maybe it’s a medical issue?) as well as being kind of physically uncomfortable for you.  Add some gender-based social stigma for kicks (real men don’t cry; women just cry to get what they want).

I think of crying in front of people like this – not a fear because I don’t think of it as an unnatural, shameful act (and because I’m generally too wrapped up in my own feelings to care about anyone else’s at the time), or as a lack of control, but it can be kind of inconvenient and frustrating.

I think that if someone makes an INFP (or anyone else) ashamed and anxious enough about crying that it does become a thing they actively fear, that is pretty dreadful.  If the INFP can’t completely avoid these people, might I suggest learning debate tactics?  Not even necessarily to argue – they just allow for some distance to pick apart the method someone’s using to be awful instead of focusing on how the words hurt.