Paul explains the words ‘but not well’ in the previous verse. The Galatians were in danger of having their affections reset in a bad way for two reasons.
Strong affections do not in themselves indicate what is good. It is only when they spring from a good motive, use good means, and are directed towards a good end that they can be said to be good. Indeed strong affections for something evil only increase guilt. Paul encourages them to think of the object of their affections and not to conclude that the strength of their feelings justified their switching to the law. Men justify themselves in their own minds by all sorts of tricks and certainly we can seal ourselves in delusion by persuading ourselves that the strength of our feelings is an indication we are right.