I play inverted. The reason is that the original Quake had inverted mouselook by default, and I have had it like that ever since. It feels natural if you imagine tilting your head backwards to look up, as if pulling the back of your head downwards.
But, as mentioned, the main reason is that many older games had inverted the Y-axis by default. This probably means that us "inverters" are a dying race..
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