"My dear children, for whom I am again in the pains of childbirth until Christ is formed in you, how I wish I could be with you now and change my tone, because I am perplexed about you!" NIV translation
Paul considered the Galatian believers to be his children because of the fact that he had initially shared the gospel with them. He was trying to get them to mature but he says that it is back to as if they were newborns again. He did not like the fact that he had to be corrective in his tone towards them but realized it was for their own good. Everyone wants to give good news to people but sometimes it is necessary to speak the truth in love even if the truth contains things that they do not want to hear.
"Tell me, you who want to be under the law, are you not aware of what the law says? NIV translation
Once again, Paul asks a question to try and get them to see how ridiculous their answer would be. It seems crazy to want to be under the law without even knowing what it says but, in a way, that is happening even more today. Many people (pastors included) want to pick out a certain part of the law and dig in their heels.
They will even go so far as to call it their statement of faith or some other religious sounding thing. They forget the rest of the law and the devil uses them to divide God's people. The truth of the matter (and what Paul was trying to get them to see) is that, if you put your hope in the law, then, you are bound to the entire law and not just a little part that you tend to like or agree with.
"For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. His son by the slave woman was born in the ordinary way; but his son by the free woman was born as the result of a promise." NIV translation
This passage shows us the heart of many of the problems in the world today. Abraham, like Adam (in Genesis 3), chose to listen to his wife instead of obeying God. The result for Adam (and us) was sin and death. The result for Abraham (and us) is Islam and the jealousy between Muslims and Jews. Isaac (the Jews) are children of the promise and all who accept Jesus Christ are added to the family of the Jews. Ishmael (and those who follow the teachings of Islam) cannot claim the promise that was given to Abraham. They are children of man's work and works do not gain you salvation.