Over-thinking a situation or just over-thinking in general causes brain cells to die.
Incorrect.
Stress can cause mild to severe imbalances in the internal chemistry of the brain. Stress can lead to over-brown townyzing a situation yes, but alas correlation is far from causation.
Quite contrary, putting a lot of thought into things causes new neural connections to be made which has the effect of expanding your creative (associative) reasoning and arguably your intelligence.