As an athlete, one of your goals is to improve your performance. One way you do this is by comparing your skills, your physical abilities, and how you train with others. And while comparing differences can help you learn, it often leads to negativity.
When I was a college golfer, my goal was to play on the LPGA Tour. I had two good friends, and we played a lot of golf together. However, I rarely beat them, and I began to doubt my ability to play on tour. As a result, I chose a different career. What I didn't realize was that these two players were really good; both were elected to the World Golf Hall of Fame!
When you compare yourself to others, you either create envy (I want what they have) or pride (I'm better). Neither envy nor pride honor God, and He is very clear about what He thinks of these sins. Yes, He says pride and envy are sins. Remember how pride got Adam and Eve into trouble? How envy drove Cain to murder his brother Abel? How pride led Peter to deny Christ?
You are uniquely made by God; you bear His image. When you compare yourself with others and are disappointed, are you not telling God that He didn't make you right? When you think you are better than others, are you loving and serving them as Jesus commands? Comparisons easily lead to discontentment, and when the disciples started the comparison game, Jesus said follow Me.