The hormonal and physical changes which a person goes through as a teenager are a major challenge in themselves. Psychological development in terms of developing identity is a huge hurdle and is a path frought with difficulty. Just at a time when identity is being formed, physical changes make the human body change dramatically which is a phenomenon that can exacerbate any insecurity and fears prompted by the psychological changes.
These, coupled with a time of great change in terms of entering the workplace or higher education, establishing and understanding relationships and reaching thresholds in terms of educational milestones is a highly tempestuous time.
I for one wouldn't want to go back there!
Besides all the challenges of puberty and crazy hormonal changes, the teenage years feel difficult because most teenagers struggle with how to cope with their feelings of rebellion while still trying to fit in with their peers. Growing up and accepting more responsibility is a big challenge, and some parents really prepare their children for this, but some kids are left to struggle through it on their own, which can lead to a lot of frustration and anxiety.
Teenage years are the hardest given people of this age are not completely aware of who they are (sense of self). Teenagers are likely to experiment with different unknowns (like smoking, drugs, and alcohol), try to find out who they really are, and learn about complex social interactions. Compounding that, puberty tends to add to the already overwhelming conflicts which teenagers face.
One reason is the amount of change and growth most people experience during this time. Teens are naturally pulling away from parents and authority figures. They are seeking to test their limits and learn to take care of themselves. This naturally leads to a lot mistakes and heartache. Teens are trying to figure out who they are and what their beliefs are going to be (not necessarily religious beliefs but just how they feel about various issues). Relationships seem to be in a constant state of flux. These changes aren't just social either. The body itself is usually undergoing drastic changes as the child moves into adulthood. Even the brain is changing during this point in a teen's life. It is a difficult set of circumstance to navigate.
As a middle aged man looking back on my teenaged years I certainly thought they were hard at the time. With the benefit of experience I now know that they were not the hardest years of my life, but they probably were the most confusing. Everything is new and at that age we often don't have a strong sense of self yet. Once we develop our belief system and confidence in who we are, life's later challenges are easier to deal with, even though they may be more difficult.
I wouldn't say the teenage years are the hardest years of every person's life, but they certainly are for some. A teenager's body is going through some drastic changes to prepare itself for adulthood. Not only are the changes physical, but mental as well. They don't think and reason like they did as little children; they start thinking more with their feelings and emotions rather than with their reason and sensibilities. Life seems more drastic and dramatic.
Wise parents recognize and welcome the changes they see in their teens and realize their little boy or girl is no longer a child, but becoming a young adult. Our teens need lots of love, understanding, compassion, and support to help them weather the changes they are going through. If we do, they will make it through those hard "teen" years and turn out to be pretty good adults.
It is during one's teenage years that one begins to assume the role of an adult, spreading one's wings, so to speak. Teenagers wish to be treated like adults, but are not yet ready to accept adult responsibility, and do not often understand the distinction. At the same time, most teens pass through something of an identity crisis, as it is at this point in life they wish to define and distinguish themselves from others, and at the same time gain the respect of their peers. Teenagers are very quick to compare themselves to others, and quite often deep seated feelings of inferiority result. Additionally, during these years, preoccupation with sex can become dominant, and often cloud one's judgment. When one compounds this with the conflicts between parents and children that naturally erupt during this time--it is nature's way of encouraging young adults to become independent--the teenage years can be very difficult and challenging. Fortunately, most live through it and eventually watch their own children deal with the same crises.
I'm not sure I accept the premise of the question, though I certainly know where it's coming from. Looking back, my teenage years were as enjoyable, and certainly as carefree, as any in my life. I don't mean to downplay the challenges and difficulties of being a teenager, which is even more complex now than it was when I was young. But I do think that too many teenagers are in too much of a hurry to grow up, and are afraid to enjoy what could be one of the best times in their lives.
It's because these are the years of greatest change and greatest uncertainty. These are the years when you are becoming an adult. You want greater autonomy, but you are still dependent on your parents. Also, you are starting to worry more about your future in terms of what sort of job you will get, who you will have a relationship with, etc. These things make the teen years hard.