Dyothelitism, the orthodox position asserts that Jesus has a human will and a divine will – the same will as the Father and the Spirit. This is necessary, because Jesus needs to have a divine will to be truly God, and He must have a human will to be truly human. If Jesus did not assume a human will, then our human wills cannot be redeemed.
The reason Monothelitism is the implicit belief of so many modern Christians is because our individualistic post-enlightenment culture assumes that a unique will is the definition of what a person is. So they assume that in the Trinity, will come with personhood, not nature. They assume what’s called Social Trinitarianism, that the Father, Son, and Holy Spirit each have their own will and that the person of Christ has one will. However, the classic view is that will comes with nature, so God has one divine will shared by the three persons of the Trinity, and Jesus Christ has two wills – a human will and a divine will. Monothelitism is also at the root of the Subordinationist heresy, the belief that the Son eternally submits to the Father. Jesus is indeed said to submit to the Father in scripture, but traditionally, that’s been seen as His human will submitting to the Father. His divine will can’t submit to the Father because it’s the same will as the Father’s will. However, if will comes with personhood rather than nature, then when Jesus is said to submit to the Father in scripture, it’s assumed to be an eternal subordination.