The trait you're describing is often referred to as being "thin-skinned" or "hypersensitive." This type of person tends to react strongly to perceived slights or criticism, even if unintended or minor. They may become easily upset, offended, or defensive in situations where others may not react as strongly. It can be challenging for others to understand what triggers such individuals because the triggers may vary and may not always seem rational or proportional to the situation.
