Fear and Greed

 Fear and greed are probably the most common causes for making bad decisions in chess. Both emotions prevent us from thinking logically and systematically, and make us do stupid things. Below is an example on what greed can do to you (I'm at the move as White, and I'm the greedy one in this game).

I have been building up for a Kingside attack, and now it's time to leap into action. I started off with 1. Bh6, seeing that the Bishop cannot be captured due to 2. Rxf6, but now greed kicked in:

1...Qe6 2. Bxg7?? Kxg7 3. Qg5+ Bg6

This was the move I completely ignored, blinded by the beautiful wins after 3...Kh7. Now White's completely busted. This is the hardest type of loss - you are just waiting for them to resign, but instead they play a move that not only saves the day, but also wins the game! The worst part of it is that you have only yourself to blame.

Instead of the disastrous Bxg7, White could have retreated the Bishop to e3 with a winning game. But, retreats are seldom on your mind in the middle of an attack.

However, the greediness doesn't stop there. Being obsessed by the capture on h6, I completely overlooked a much better continuation from the diagram:

1. e5! dxe5 2. Bxe5 Nh7 3. Bxa8 Rxa8 (or 1...Ne8 2.Rae1 Ne6 3.Bxh6) and White is winning.

Takeaway

The next time you're overcome with fear or greed, try to take a moment to calm down. It's not easy to be objective is the heat of the battle, but if you succeed, it could save you the game.