When to throw an exception?

Question

I have exceptions created for every condition that my application does not expect. UserNameNotValidException, PasswordNotCorrectException etc.

Answer 1

Throwing exceptions causes the stack to unwind, which has some performance impacts (admitted, modern managed environments have improved on that). Still repeatedly throwing and catching exceptions in a nested situation would be a bad idea.

Probably more important than that, exceptions are meant for exceptional conditions. They should not be used for ordinary control flow, because this will hurt your code's readability.