We experienced the same behavior with our eldest, starting out in soccer / baseball as well. As has been previously mentioned, use your child's intellect to advantage by examining the concepts of "fair" and "equal" as well as how much in life isn't fair or equal and that will always be. Perhaps the the thing that helped the most is to help our child realize that while we can't control the whole world and make it fair, what we can control is how we react and deal with things being unfair and when they are, to what degree and whether it justifies an immediate response, a thoughtful measured response, or if the unfairness is minor enough to simply let it pass without response.
If it helps any, my eldest is now a college Freshman and has thanked us numerous times for teaching him "Life isn't fair" and how to deal with it as he learns that it only becomes more so as one ages.
