I don't want to go to soccer anymore
This time of year the newness of the school year is over and the novelty of activities is waning and sometimes our kids inform us that they don't want to continue with an after school activity like soccer, karate, music, ballet, etc. Since most of the time we are already committed financially to those activities and want our children to follow through, we get frustrated and try to convince/bribe/force our kids into going.
Instead of getting angry, it's important to identify what you are upset about: that you want your child to learn about commitment, that you don't want the other parents and kids to see your child as a quitter, or perhaps that the team won't have enough players without your child?
One option to resolve this is that you can explain that the child has to attend the activity, but he can choose to participate or just sit/watch/cheer the team. We call this parenting tool freedom within limits. The child is not choosing whether to go or not, and has choice over how he/she will participate and at what level.
This solution reduces the risk of getting angry or hurting your relationship with your child. Through the process you are teaching him the importance of being part of the team and honoring his previous commitments in a respectful way.