There are equal forces on the chess board initially but not all outcomes are draws. The side that wins is the one who make penultimate mistake. It wouldn’t be too much if I say that chess is game of mistakes. In order to win you need not make many brilliant moves but rather avoid mistakes as much as possible, which is the major hurdle of your chess progress. To avoid such mistakes I have tried to dig-out the causes of them.
A Lack Of Serious Chess
I have observed that at club level (particularly in my city and the surrounding areas) people don’t play chess to improve their game and therefore they are not serious while playing. If you are also a part of that environment you will develop the same habit, and as a consequences your thinking will become impulsive and you just play for threats or play move by move. Impulsive thinking is a side effect of too many blitz games.
These often arise when you do lots of preparation and are working very hard to improve. For example you do daily exercises for a longer period of time in order to improve your tactical play and you learn so many new things so that you are now expecting to perform better in an upcoming tournament. But actually knowledge needs time to digest.
Your expectations make you more emotional unstable as a result you make some grave mistakes which were not part of your game earlier. The simplest way to avoid such mistakes is to become more realistic about what you can achieve.
Too Much Excitement
This normally happens while facing a higher rated player and reaching a good position, or even you find some splendid move. Your heartbeats get faster as a consequence and you miss some very simple winning move. You can avoid such mistake by doing some deep breathing exercises.
By checking it carefully, you will find that these are behavioural mistakes which are result of our habits. Here I have not included mistakes that occur due to lack of knowledge as chess experts have already written a lot about this.