If you are an avid competitive gamer, then you would understand the frustration of losing after pouring your heart and soul into your game. That frustration can easily be tipped over into anger once a fellow player says the dreaded words " it's just a game".
This might not be the case for gamers who play their games casually and are in the game for the pleasure of just playing it. For competitive gamers that want to better their craft and move forward towards getting better, they invest heavy emotion into the game. Investing emotion into the game has one of two pay off. The first being the satisfaction increase of winning, and the second being the frustration of losing. This is something that is not felt if you are just playing the game casually.
This same principle can be said for any sport that is played by a competitive player. Tyler "Ninja" Belvins highlights this principle in his latest Tweet, saying, "The phrase "it's just a game" is such a weak mindset. You are ok with what happened, losing, imperfection of a craft. When you stop getting angry after losing, you've lost twice. There's always something to learn, and always room for improvement, never settle."
While Ninja doesn't mention in his Tweet that games can be played casually, as a professional gamer and someone who is one of the most successful streamers out there, he would certainly know this. Many people are grilling Ninja for saying that you shouldn't get angry after a loss, but you have to remember this opinion is coming from someone who is a competitive gamer. Ninja has built his career on competitive gaming, and sometimes when you throw your emotion into a game and receive a loss, the result is anger. It's ok to get angry, it's a natural human response.
It's not a matter of whether or not you should get angry as a competitive player, it's a matter of how you maintain that anger and how you channel it into bettering yourself. Anger can be extremely useful, and representations of that can be seen across almost all competitive sports. On the other side of the coin, anger can also be a player's downfall, as they can get lost in it.
All emotions are responses to the amount of time and energy invested into that specific thing or activity, and to just ignore emotional responses isn't a healthy choice. It's a matter of how you gauge those emotional responses and take them into your next game, not the initial emotional response.