The answer to 'why is my mosquito infestation' is that mosquito infestations are typically caused by standing water around your home where mosquitoes breed. Common symptoms include being bitten frequently, especially at dawn and dusk, and seeing lots of mosquitoes flying around. To solve a mosquito problem, you'll need to eliminate standing water sources, use repellents, and potentially treat the area with insecticides.
Mosquitoes only need a small amount of standing water to lay their eggs, so check for things like clogged gutters, flowerpots, birdbaths, and even puddles after rain. Removing these breeding grounds is the best way to get rid of mosquitoes long-term. You can also use insect repellent and cover up when spending time outdoors. For severe infestations, you may need to apply insecticide treatments, which is often best left to professional pest control technicians.