The on readings originated in China and to Chinese sound rather familiar, but the kun readings, representing native Japanese words, are completely unrelated to the character's pronunciation in Chinese. And there is no rule to follow in knowing which reading to use. Mixing them up not only creates communication problems but may lead to ridiculous results.
問題在那些字念音讀，那些字念訓讀，日文裡並無規則可循；這就好像中文裡的「解元」（中國古代官名）讀ㄒㄧㄝㄩㄢ，不念ㄐㄧㄝㄩㄢ一樣沒道理，但若弄錯則大至影響溝通，小至鬧笑話。