Why are you treating all Japanese like a monolith and not a society made up of different types of people with different personalities.
How one person writes a character is not a collective hive-mind of the thoughts of all Japanese people. And even as a creative, not all characters are a 1-to-1 reflection of yourself.
If a lot of the Japanese people you have met have been low energy, it could be possible it was because 1) cultural/language barrier which can effect how people interact. or 2) they were a professional and had to be serious for their job.
Someone that you are besties with and are more fluent in English is going to act differently simply because their is more trust and better communication.