Why would meeting an alien give most people PTSD?

Have you ever visited a culture completely different than the one you were raised in? Imagine being raised in a different religion than the country you are going to visit or imagine yourself being atheistic in a really devout religious country. This could be very interesting or a very dangerous experience depending upon how sophisticated and evolved you are in dealing with every situation you encounter. Because when the differences are too great knowing what to say or how to act can be very dangerous if you say or do the wrong thing. Acting wrongly or in a perceived wrong way might cost you your life or freedom.

So, what if you meet a being and you don't know the rules at all? Do you pull out your gun or knife only to be cut in two by a ray gun? Or does this alien come inside your mind with you and talk to you so you cannot tell who is saying what or who you are and who they are because you aren't used to dealing with all this and because of that have no boundaries in your mind set up for this kind of thing?

The potential outcomes are without limit in a world different than you know the rules for. Because of this if you aren't an intuitive who senses the motivations of the beings you are around you could easily panic and have a psychotic break and never return to reality ever again as long as you live which might be a few seconds or less if the being or beings feel threatened by your behavior or behaviors at these particular moments at your first meeting.