When I'm overconfident and in a winning position. I blindly make one terrible mistake and everything goes against me.
Why you lose at chess
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When I start rushing my moves, playing faster than normal then I lose. This is evident in my quick and blitz rating compared to my long rating.
All of the above...=P
You lose because you cannot win all the time.
I make one knee-jerk reaction move without checking all the possibilities and the game is over. Often what I over look is very obvious... after the fact.
I lose at chess because what is the fun of winning every game. Although, I could easily do this there is no reason to win every game. It is important to keep your opponent guessing whether or not they will win or lose. Nobody would want to play me if I never lost and I love chess... 
I lose because sometimes I play too quick and don't see the right move, and sometimes because I play too cautious and give my opponent too much of an advantage.
Because my opponent is better than me. :-)
i regularly make moves while at work both early in the morning and late in the evening as a short distraction from the task at hand. I can think of numerous times when i have been crushed by a response that i should have seen if i had taken the time to really look at the position. of course, i am outplayed quite often too but those dont sting as bad as the "doh!" moments as previously mentioned.
I also catch myself pseudo analyzing..meaning that i see a move that i prefer and then i sort of skim the other possibilities without considering them or the other top candidate moves as clearly as i should then make the move that initially appealed to me...just laziness in these cases, sometimes it pays off and my instincts were right, more often it is disastrous
I lose because I tend to move when I see something that will give immediate results rather then thinking out a plan. I don't look into all positions I should be and analyzing the board before moving.
I loose ; because i play lte in the night ; so tired ; sometimes there is TV ; or my wife distracete me ; i play also in my bed with laptop; at all its concentration ; and of course my opponent is also better and very ceative and strong ; i have to prtactise more
I know why i lose.
a. i get out played. (im not that good)
b. playing so many games at once some times you forget about that extra knight guarding the queen or forget that bishop in the corner.
either could be fixed if i took every game as super duper important and took my time. and check and rechecked every thing...
I cannot speak for you, but I know the cause of some of my losses.
This morning I woke at 5:30am--the usual time. First I processed some credit card orders that came in overnight, then made a couple of moves in games here, then went to fix coffee. The hopper of my burr grinder needed more beans. I opened a new bag and poured them straight into the French press, instead of the grinder. Silly, absurd behavior.
Why would I do this? Fatigue.
My brain needs the coffee this morning so I can survive another day of mental effort. But, I've already made some moves, and haven't yet drunk that first cup. Were my moves sound? I'm afraid to look at those games.
Playing when I'm incapable of mental effort causes me to lose chess games.