@99mbd TL;DR~ Reasons: Survival: Mosquitoes bite us coz of their survival instinct Consider a random dog from the streets in the place of a mosquito-- -> If it tries to bite you, you have 2 options - fight or flight(run away) -> If you choose the first option, you realize that you're taking a risk coz a dog's bite has a better chance of being fatal than the puny mosquito; -> Taking the second option means you don't hurt em and leave em as is, so most of the time people tend to take this option as the easy path with the least risk, or you could just scare them away. Threat to species: What would we do as a species if mosquitoes are listed as endangered? We'd naturally try to protect them, even if they try to bite us, we'd even try to feed them with blood in a suitable form. Contribution to the environment: Animals like dogs can be trained to serve and/or guard people Small insects or should I say, "pests" like mosquitoes do more damage than serve any "purpose" to people. So, as- I) there are more mosquitoes in the world than people and killing a mosquito doesn't affect the environment in any way, and II) since its been allowed by our sense of morality, we kill mosquitoes and cockroaches while preserving dogs. P.S.: Cows are killed as well duh