Like Walt said, and also not ready emotionally or with maturity .

I really like the way the UK does it. They have a year between high school and college to help with the transition. Many, if not most,  do organized volunteer work and this gives them strong  guidance and pride, and helps mature the emotions … Read more

The most common reasons people drop out of college, in my experience:

They weren't prepared for the workload
They discovered they didn't want to pursue that career path
They thought college would be all partying
They weren't prepared for college-level classes, by their high school education