Cassius, I want give you credit for your mentioning the following idea:
The phrasing at the end of fragment 1, "pleasure / pain" is not being differentiated from "desire for pleasure / pain" so he must be talking loosely. If you feel pain there is always a reason for that pain, and the desire to remove that pain -- so that is how we get either groundless or natural
So this wording on the wall is a quick way to wrap up the whole idea quickly.