Paul had developed an illness, most likely involving his eyes. If he hadn't gotten sick, he probably wouldn't have preached to these people. He needed healing and they needed the gospel. They welcomed him as if he had been Jesus. They were so joyful and filled with love for the one who brought them the gospel; Paul knew that they would have torn their eyes out and given them to him. They were believers because of Paul's message. They were sons of God because of their faith. They had been given the Spirit because they were sons. The Spirit in them cries "Daddy! Father!" and they know they are no longer slaves, but they are not just free. They are children of God and heirs of his promises! Of course they were joyful! Joy is fruit of the Spirit!
Now, however, they have rejected the message of faith! They turned from justification by faith to justification by circumcision! They are trying to attain their goal by observing law. They have put their faith in human effort rather than trusting in the power of the message that had been clearly preached to them: Christ was crucified for you!
When they turned from their faith, they lost their joy. Turning back to an emphasis on human effort turned them from people who would have torn their eyes out into people who wanted to tear his eyes out. Paul knew that their hostility was a result of their change. "Have I become your enemy because I tell you the truth?" Paul asked.
Nobody hates the messenger of grace more than the person whose focus is on his own effort instead of the justifying nature of faith in Jesus. When you turn to slavery, joy is gone. Stick with faith and joy remains! That's Life at Work!