"What has happened to all your joy? I can testify that, if you could have done so, you would have torn out your eyes and given them to me. Have I now become your enemy by telling you the truth?" NIV translation
The Galatians had loved Paul so much that they would have even been willing to give him their eyes but that has changed. Paul asks them a question to get them to see how ridiculous their behavior is now. Because he loved them enough to tell them the truth, they turned on him. In our world today, it is much the same way in that many will just go along with whatever in order to keep the peace. They will say that they are doing so out of love but we must ask ourselves if that really is love or is love warning someone about a dangerous condition. Paul's love for the Galatians required him to tell them that they were turning back to be enslaved again by the law. Are we ready to share the truth with people that we say we love? That is the question that we must each answer in our minds but we must also remember that our answer has eternal consequences for ourselves as well as them.
"Those people are zealous to win you over, but for no good. What they want is to alienate you from us so that you may be zealous for them." NIV translation
Paul always preached of grace but others were trying to put a little bit of law back into it. They were not doing so to benefit the believers but were doing so to further their own goals. The same is true today in many places for example: If you go to just about any church for a year, you can bet that you are going to get at least a month long focus (if not a sermon series) on tithing.
In many cases, this is done at the end of the year to make people feel guilty about their financial contribution so that they will give more. There are many other examples but our reaction should be the same as what we see here in Paul.
"It is fine to be zealous, provided the purpose is good, and to be so always and not just when I am with you." NIV translation
Paul is talking, here, about people that are playing both sides of the fence. When he was around, they would be sold on grace and their freedom in Christ. When he was away, they would go back to trying to set up another religious system. It is sorta the way it is with many believers today that will attend a place of worship once (or even twice) each week but when they are outside of that building they are totally different. This is the same type of thing that Jesus warned the church of Laodicea about in Revelation 3.