Why do people die unexpectantly from blood clots?

So, I have a weird fear about dvt, and dieing from one. Ive worked on it with a counselor, and thought I was over it. Last night I heard that one of my friends moms died in the shower from a blood clot over the weekend. I havent really heard much information, other than it was unexpected.So im not sure if she had any warning or not. But why do blood clots not show any symptoms??Ive always been confused on that, cause Drs, tell me that most people do have symptoms... so can anyone help me... im very scared im going to die from dvt. Even though im 22 and no hereditary or pre exsisting conditions!