I always cringe when I hear a parent proudly proclaim that they are 'friends' with their child. I wonder what they think this means. You are not one of your child's peers; you probably don't share their interests, or at least not at the same level; you are almost certainly not stimulated intellectually or emotionally by the same things. To strive to be your child's friend is fraught with danger.
The truth is that children really don't WANT you to be a friend in that way. Your child needs you to be a parent, even when they might be screaming to the rafters that they hate you for some boundary you've set. Children without boundaries are scared children, and one of those boundaries needs to be around you; teenage children whose parents are intent on being their 'buddy' will learn how to manipulate that relationship very quickly, and will resent you trampling on their territory to boot.