When combined, the logic is very straightforward: 不 (not) + 高 (high) + 兴 (spirits) = “not in high spirits,” which is a perfect description of being unhappy.
While traditional Chinese culture often values social harmony (`和谐, héxié`) and discourages overt displays of strong, disruptive emotions, 不高兴 (bù gāoxìng) occupies a unique space. It's considered a relatively mild and acceptable expression of negative feelings, especially within personal relationships (family, close friends, romantic partners). In a Western context, saying “I'm not happy” can sometimes be a very heavy statement, implying a serious problem or a precursor to a major conflict. In Chinese, 我不高兴了 (Wǒ bù gāoxìng le) can be much lighter. Depending on the tone and context, it can range from a serious expression of dissatisfaction to a mild, almost childish pout. It's less about a deep-seated existential sorrow and more about a temporary, often externally-caused, bad mood. You're not “sad,” you're just “not happy” right now.