The way our bodies react to mosquito saliva motivates us to avoid being bitten. Which must have had evolutionary benefits, keeping us away from diseases.
I.e. all those people that didn’t mind them and never got itchy from mosquito bites appear to have died out. And mosquitoes really wish that wasn’t true.
I literally swatted a mosquito the other day because it bit me while I was doing something else. I didn’t feel it land, but I did feel the bite which triggered me to slap it out of existence.