Emotion words and expressive strategies cannot, as a rule, be used in reference to persons other than the speaker. In cases where they do refer to persons other than the speaker, most must either be used in quotation or must go through grammatical manipulations that explicitly mark them.
This is particularly true in the case of adjectives of emotion, for example, kanashii, 'sad'. Now, Kanashii 'I'm sad' is grammatically correct, but *Ano hito wa kanashii 'He is sad' is not acceptable under ordinary circumstances. Instead, what is acceptable is Ano hito wa kanashi soo da 'He appears to be sad'. In Japanese it is necessary to mark the sentence if it is not about the speaker's but about someone else's emotion. The reason for this is that, although one can experience one's own emotions directly, someone else's emotions are not so accessible.
The distinction in Japanese is not necessarily required in other languages. In English, for example, it is possible to say "I'm happy" and "I think so" as well as "Yamada is happy" and "Yamada thinks so." It is important to remember that in Japanese, the distinction is obligatory, and extra attention must be paid when referring to one's own or another's thoughts and feelings.
Senko K. Maynard, Expressive Japanese: A Reference Guide For Sharing Emotion And Empathy