Why do mosquitoes suck blood when most other insects try to avoid people? I really don't know, but I have made an uneducated guess. I think the original (or ancestors of) mosquitoes sucked the juice of fruits. Some of the mosquitoes came upon a fresh carcass of an animal with soft skin, and they discovered a way to suck blood.

I think it's different for the parasitic worms. It's easy for their eggs to get into a much larger animal. As they hatch and grow inside it, they developed a taste for larger animals. Meanwhile, mosquitoes start life away from humans. I think they developed the courage to approach living humans when their ancestors sucked a fresh carcass.

Another thing that's curious about mosquitoes is that they start their life in water. When they get older, they can float on water with their legs. I have no idea how that came to be.

There are some mosquitoes in my apartment at this moment. I saw them get in when I opened the door.