Why do people get anxiety? or some people are shy? it's not like i want to be but i am?

I always try to get out of my shyness but as soon as someone i'm not familiar with comes around i get shy and my voice crackles and i think i get a bit of anxiety. i try to snap out of it but it always catches me. i don't like giving public speeches or talk in front of a large crowd. i get so nervous and my voice becomes week, i feel a bit intimidated. around the people i am comfortable around i act totally different. why is this so? why do i get anxiety around random people and how can i fix this?