why do we emphasize the emotional response is worship?

I have done and I have heard other do the same - we evaluate the worship during a church service by our own or others' emotional response to it. We evaluate worship by how much energy the music had or how many people responded publicly with a request for prayer or with tears. There is nothing wrong with any of these being a part of our worship, but they should not be how we evaluate it. God can be just as present in a service with none of that as in a service with it. And He can be just as absent from a service fill of emotion as from a service with no emotion. Our emotional response is not an indicator of how "good" a worship service was.

"Each time we meet to worship the triune God, He should be the all-consuming center of our attention and affections. His greatness and splendor should become bigger in our minds, hearts, and wills. His desires and commands should become more precious to us. Jesus Christ and His atoning work should become more glorious and amazing to us." (Idolatry on Sunday Mornings, Pt. 2, Nov 30/05)

These things can involve an emotional response, but it is not required. The important thing in evaluating worship is that God was glorified and exalted through that time spent together singing.