If you are upset for some reason, there is something in you already which is not at peace. Some part of your mind is identified with ego, having an emotional reaction, or perhaps something is coming up for release.
If you are then exposed to a person whose behavior seems to be in some way provocative, a potential attack, or something that your ego does not like, you are much more likely to experience that person as causing you to be upset.
Or rather, because you're not on a firm footing of certainty and peace, and your mind is unhappy, you are much more susceptible to your pre-existing upset coming up in your face.
When you are upset your ego sees itself as vulnerable, a victim, under threat, and being caused from outside. It is then very easy to slip into that kind of perceiving where you think the other person is doing something to you.
And since you're already upset, it will cause your pre-existing upset to come up even stronger. You'll think they are pushing your buttons. They sort of are, but the result of pushing the button is that YOUR stuff comes up inside you, not stuff that THEY put there.
When you are upset already, other people will seem upsetting, and you will react to them. When you are not upset already, you will be much more detached and it is far harder to be provoked.
Every time that I am already upset for some reason, if someone does or says something *potentially* upsetting, I am much more likely to have reactions of victimhood, where I'll start to quickly project my shit onto them, treat them poorly, and react to them like they're doing something to me.
But later, if I'm honest, I'll recognize that I wasn't really upset with that person... I was ALREADY upset. My upset wasn't their fault. I reacted poorly. I was not clear about the truth. I had slipped from a firm footing onto a slippery slope, and had become confused about who was doing what to whom.
I notice this again and again. If I am upset already, I seem to be more upsettable, and I'll think it's someone else that's wrong. Yet if I'm not upset, the person can do or say the same things and it has no effect.
It is the hurt ego which reacts. It's the hurt ego that sees itself as vulnerable. It's the hurt ego - the pain - that responds with attack, perceiving attack where there really is none.
If I had no upset whatsoever and were perfectly at peace, nothing would be ABLE to upset me, because I would have nothing that is already upset to react WITH. That is invulnerability.