Paul is hard on the Corinthians because they were grownups still acting like children. There were petty jealousies and envy. In 1 Corinthians 3:3, Paul says that the believers were still living according to the dictates of the flesh. 
He writes, “…because you are still fleshly. For since there is envy and strife among you, are you not fleshly and living like ordinary people?” He calls them spiritual babies. The reason? They were a divided church, unable to work together. The verses following verse 3 show the source of their division. Some people were attaching themselves to Paul, some to Peter and others to Apollos. Their childish preferences kept them from doing what God wanted them to do: Work Together for the common good!
The independent commission investigating the September 11, 2001 terrorist attacks reported that the rescue efforts were hampered by a “Rivalry between New York’s police and fire departments.” Their report said, “This rivalry has been acknowledged by every witness we have asked about it.” Americans needed cooperation, not competition on that gloomy day. We are at our best, when we set aside our own ambitions and throw all our efforts into the common good, instead of putting energy into unhealthy rivalries. Surely we must see how important this is for the church of the living God. In 1 Corinthians 1:10, Paul exhorted the believers. He said, “Now I exhort you, brethren, by the name of our Lord Jesus Christ, that you all agree, and there be no divisions among you, but you be made complete in the same mind and in the same judgment.”
J. L. Wilson says, in his sermon entitled better together, “I’m so glad that the Corinthian Church had this problem, because it allows us to have this beautiful teaching that Paul gives. Paul makes it clear that he and Apollos were partners not competitors in ministry. They are just doing their part in the Kingdom work, it is God, not them that gives the growth and is deserving of the glory. The verse that follows this explanation is precious. It says, “For we are God’s co-workers.” This is a very comforting truth for us all. We’re all in the same boat! We’re all in this together. We are much better, much happier, much more effective, and much more efficient if we expend our energies in cooperative efforts.