Do some fish smell "fishier"? Why?
Certain people who are "super smellers" who have a heightened sensitivity to smells. Assuming you aren't one of them: fish smells "fishy" when it's starts to age. If you can find a reputable fish monger, that would help.

That said, if you're sure you have fresh fish, and some fish just simply smell more, you can cut down on the smell, by a quick rinse and a marinade with an lemon juice.

As for why fish smell fishy... it has to do with amines contained in the fish. Amines get released as bacteria work to release them. Salmon, Tuna, others... are higher in amines, while fresh white fish, tend to be lower. You'd have to do a bit more research to find out more specific species...