There was a deleted subplot involving Decker's interest in spritual unity and transcendence, which was part of what had motivated his interest in Ilia and the telepathic unity of Deltan lovemaking. V'Ger gave him the opportunity to experience that kind of transcendence. Unfortunately, that was all lost.
I like to think that Decker's prime motivator in joining with V'ger was the whole saving billions of lives on Earth, over it being mostly about some kind of personal journey thing for Decker.
McCoy: " Well, isn't it lucky for you that we just happened to be heading your way?"