Yes legally a police officer has to tell you why they pulled you over.
Most police officers will tell you why they pulled you over when they come up to your vehicle but if not then ask them why they pulled you over.
If they do not tell you why then they are violating your rights and they are violating the law.
I've had police even ask me if I knew why they pulled you over but you should always tell them no you don't know.
Police cannot just pull you over for no reason as the police have always got to have a probable cause to pull someone over or stop them.
If the police have no probable cause to pull you over then they can be in trouble.
Also never allow the police to search without a search warrant because they may plant something illegal on you and then say it's yours.
That has happened to some people before.