So I was reading A Dictionary of Basic Japanese Grammar, and in the part where it explains the uses of いる as an auxiliary verb after て it says something like that: "(...) Also, verbs like 言う and 思う with a third person subject require the ~ている pattern if the sentence expresses a present state." And then there's this example sentence:
ウエストさんは日本語がやさしいと思っている。
My question is: why does it have to be 思っている? And can I use 思っている as well when speaking in the first person, or is it wrong?
This seems to be "just a rule", there's some discussion of it at this question.
Some surrounding thoughts to think about though. Non-stative verbs in the plain form are interpreted as talking about the future, unless the action is generic or habitual. ジョンが食べる means either "John will eat", or "John eats(habitually/regularly)". To talk about an activity that continues, you switch to the stative form ～ている. (See Derek's excellent coverage here)
In the case of 言う or 思う, the non-past seems to have an implication of "I (typically) say" or "I (tend to) think". Moving that to the stative seems to pick up an implication of "I (obstinately) say ...", or "I'm (stuck) thinking...". It seems to be polite to emphasize the ephemeral nature of your own statements or thoughts.
Confusingly, the structure that implies boorishness in oneself is seen as a virtue of consistency in someone else.
