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?