I came across this fact "If a strategy is strictly dominated, then it is never a best response" in chapter 6 of Steven Tadelis' Game Theory An Introduction.
It is remarked as an obvious fact, but I couldn't think of why.
According to the definition of strictly dominated, this tells me the payoff of the strictly dominated strategy (pure) is less than the payoff (or expected payoff) of the dominant (pure or mixed) strategy under all possible pure strategies of the other players.
How does this lead to the strategy being not a best response to any mixed strategies of the other players?
Edit: I figured out why so.