Parents may feel that the school system doesn't challenge their child or approve of the curriculum taught. Homeschooling is the easiest decision to make. The hard part is to convince friends and family that the right decision has been made. There will be a lot of criticism. There is no doubt about that.
Socialization is a big issue.
The child will be socialized with a broader range of friends of all ages than he can get at school. He will learn how to talk respectfully to adults and how to help out those who are younger. He will have the freedom to learn at his own level and to pursue a subject until he has learned everything he can about it. Once the decision has been made, go to the library and read. Read all the books, magazine articles, everything you can find on homeschooling.