The answer to 'what causes mosquito' is mosquitoes are attracted to and breed in standing water. Mosquitoes need water to lay their eggs, and they can breed in even small amounts of standing water like bird baths, gutters, and pet bowls. Mosquitoes are also drawn to warm-blooded hosts like humans and animals, which they feed on for blood to nourish their eggs.
Eliminating sources of standing water around your home is the most effective way to reduce mosquito populations. You should also use insect repellent and wear protective clothing when spending time outdoors, especially at dawn and dusk when mosquitoes are most active.