While high school is not nearly as brutal as middle school is, it can still present a young person with a vast suite of different challenges which they need to overcome if they are going to get anywhere in this world. After all, there are a lot of different, tricky factors which every high school student is going to have to navigate around if they are going to do well in the later parts of their lives. And while it might not seem to be a big deal to be very socially adept or to be a great athlete, such things can actually work to your advantage later on in life. While grades would at first appear to be the most important part of your high school career, they are only one part of a far larger whole which a lot of people do not put enough thought into properly enriching.
For instance, being a very sociable, amicable person is one of the biggest advantages that you can ever have in this world. While some professions (such as in medicine) permit a person to be an absolute jerk so long as they are highly competent, for the most part we are all sales people. And if a sales person is not friendly and at least fairly charismatic, the sale is not going to be made. This is a crucial part of getting dates, marrying, and conducting almost any kind of business that ranks above the simple act of buying yourself some groceries.
You might also consider how your success in your grades during high school can impact your future. While grades are not the entire picture, they certainly do tell a story. For some people, it is a story of only studying the night before a test happens and rarely turning in their homework. For some other people, it is a story of being driven to do their best in every subject they possibly can. Combined with the extracurricular activities you do, your grades make a major impact on how colleges (and possibly even potential employers) view you later on.