Why can't teachers befriend students?

Too dangerous to have such close relationships. Students are not mature enough, they often misunderstand actions and may interpret interest in friendship as something more. It is for the protection of both the student and the teacher. It's sad, but that's how it is. What is also sad is how teens take advantage of personal relationships - try to be friends for better grades. And once the line of friendship has crossed, there have been numerous accusations made against teachers, many false, but sadly sometimes true. And the worst is that students get crushes, and well, on very very rare occasions teachers get intimately involved.