Thinking too much can cause us to be nervous, worrying about things, feeling lonely .
Because your brain tells your whole system to be nervous.
Instinct, I guess. If we were never afraid, our species would have died out centuries ago. Being cautious is a survival thing, really.
An early precautionary system I guess
There are many reasons to be nervous, like when we are not happy, when we worry about something or someone, when we have quarrel with someone, anyway, we get nervous, it means we have problem not be able to solve.