halacha - If a person stops doing the bad action or begins to do the right action, why does he still need to do Teshuvah for a חטא (sin)?

If a person stops doing the bad action or begins to do the right action, why does he still need to do Teshuvah for a חטא (sin)? In other words, why do you need to do Teshuvah if you have stopped the behavior?