There's nothing to get, it just is. It is part of one's experience like feeling gloomy on a rainy day (like today :-) "Rainy days and Mondays always get me down" is information; not everyone responds that way.

Sounds like you have figured out that a particular person triggers this state in you. That too is good information about yourself. Now you can explore that further and "experiment" and learn what's going on and perhaps figure out how to change it so it does not happen (if you do not want it to happen) or avoid the person, etc. I'd say that was potentially very "productive"?