A certain amount of anxiety is natural, normal and necessary. All creatures get anxious. Anxiety serves a couple of purposes: it motivates us to do things we must do (“Uh, oh, I forgot to pay the light bill!”), and it keeps us safe (“There’s a shark! Get out of the water!”).
But some people have an uncomfortably high level of anxiety. This can be genetic, something you inherit like your eye color. It can also be learned, because when you were growing up you saw other people responding in highly anxious ways to problem situations. Or, it can be in response to a difficult event or situation that has triggered your anxiety. Panic attacks can start when you’ve had a high level of constant stress and/or anxiety for a period of time.