"Formerly, when you did not know God, you were slaves to those who by nature are not gods. But now that you know God - or rather are known by God - how is it that you are turning back to those weak and miserable principles? Do you wish to be enslaved by them all over again?" NIV translation
Before the Jews had came to faith in Jesus, they had been under the high priests and other religious leaders who were mere men. After accepting Christ, they were freed from that religious system and brought into a right relationship with God. Now, some of them were allowing the religious system to creep back into their lives and bind them to the law once again. Paul asks them these questions to get them to see how ridiculous it is to be set free and then turn back to bondage. We too are set free from all of the rituals and traditions when we accept Jesus but we must also be ready for those leaders who would try to put us back under the law.
"You are observing special days and months and seasons and years! I fear for you, that somehow I have wasted my efforts on you." NIV translation
Paul uses some pretty strong words to get them to see how dangerous it is to try to mix law with grace. People were trying to put them back under the Sabbath restrictions (which were a part of the law) and once you add a little bit of law then you no longer have grace. We are adopted in God's family and become "sons of God" by His grace and not through our own efforts.
"I plead with you, brothers, become like me, for I became like you. You have done me no wrong. As you know, it was because of my illness that I first preached the gospel to you. Even though my illness was a trial to you, you did not treat me with contempt or scorn. Instead, you welcomed me as if I were an angel of God, as if I were Christ Jesus himself." NIV translation
Paul had a pretty nasty eye disease in which excess pus ran out of his eyes. This would have been tough to look at and "a trial" to show him kindness and love. We see, here, that it was because of this disease that Paul had been able to tell them about Jesus. They welcomed him instead of shunning him and that is what we are called to do with the less fortunate and those that the world looks on as being lower than the rest.