someone please tell me why?

well what bug are you talking about? mosquito? well here then:

A mosquito doesn't actually bite you, of course. It sucks your blood.

To help enable effective blood-sucking, it first injects anti-coagulant saliva to stop the blood from clotting or forming a scab while it feeds. When the mosquito goes away, its saliva stays in the pocket under your skin.

Now comes the itchy part. Your body releases histamine to fight off a foreign substance. It's the same as an allergic reaction. The histamine causes swelling around the area and as a side-effect it itches.

bcuz the bug inserts some of its saliva to clot the blood so it doesnt squirt everywhere. everyone has an allergic reaction, so it may itch differently for some. i hope this helps! =]

Becuase they have poison. Or they are dirty. Its not enough to hurt you unless you're allergic. So it kinda iritates your skin. All people that itch are allergic just maybe not as much as others. Thats the people that have to go to the hospital when they get bit.

the Silvia from the bug enters the body which makes the white blood cells attack it which causes red bumps and itchiness.

because there gross and leave pisoinous stuff that makes you itch