Black spots/lines in my teeth?

There are black lines and spots on the bottom two far back on both sides they looked like cavities so I went to get it checked out. The dentist told me they were only stains and to brush harder and they will come off. I don't eat sweets or drink dark liquids except tea and rarely wine. I have no pain or doscomfort so I know it's not cavities. Theve been there a good year. I brush everyday too. I went to an oral surgeon yesterday and he saw them too and told me I need to scrub harder LOL. How do I get them off?!? I've been scrubbing ferociously. I have WHiTE teeth except those areas wthell! How do i get them off