Your game tends to follow your emotions. When you feel great about your tennis, you tend to play better. If you feel unconfident about the way you are striking the ball, you tend to play worse. One fact that seems to confuse even good players is having positive emotions should guarantee great tennis, maybe even guarantee winning. It doesn't work that way. Positive emotions only increase your odds of winning, that's all you can hope for at the end of the day. This is why when Mr. Meathead makes a concerted effort to be positive amidst the stress of a tennis match and things start to go south, they start to believe having emotional control has absolutely no value. They revert back to their barbaric ways, slamming balls into the fence, semi-tanking by going for the outright winner, berating themselves after each point, and just having a "Why me?" attitude. When has Mr. Meathead ever had a brilliant idea.
Instead, a much sounder approach is to have no feelings one way or another after each point. This might sound kind of ridiculous, but it works. On the pro tour, the 20 seconds between each point is used purposefully to rid themselves of poor thoughts and replace them with thoughts of optimism. Now, I know what you are saying, "shouldn't I jump up and down after I hit a great shot?" The normal person would do this, but again if you watch the best players in the world, they use momentum and opportune times to give the occasional fist pump...like at 4-4 after breaking serve.
The whole idea behind not reacting to each point and having no emotion is to prevent an emotional rollercoaster. If there is a high, there will be a low. An over-celebratory fist pump at 1-0 in the first is going to be followed by a racquet "ding" on the cement if things don't go your way. Very common at the lower levels of the game. The good to great players have seen this movie many times and don't want to sit through another episode of Debbie Downer.
Think about it, in a close match you will lose every other point. Reacting after each point will force you to go up and down emotionally, very exhausting after a closely contested 2-3 hour match. The truth of the matter is no matter how many hours you dedicate to your tennis, you are a human, and you will make errors you have no intention of making. Accidents happen, just move on. Reacting after each point temporarily throws you off balance emotionally and that time could have been better used thinking about way to be more productive against your opponent. Mental fortitude is a limited resource, don't squander it. Lastly, you don't want to overemphasize particular points, try to treat them all equal.
Now this is why when your coach starts to talk about your footwork, follow through, fitness, etc- all great things worth exploring. At the heart of the matter is how did you compete? It's frustrating to see coaches/players (I'm certainly guilty of it, but trying to be aware) focus on the wrong during competition. American players don't lack the talent, they lack the mental maturity.