While Nimzowitsch gained notoriety for this outburst, most of us can sympathize with the frustration.  What are some reasons we lose to players who aren't as good as us?

This happens:

Overconfidence, careless play, some physical factor such as lack of sleep, one gross blunder, thinking about something other that the game, etc. Finally, who said they didn't just get better than us?

Or this:

psychology: If you are better rated, you might get sloppy if you play someone you should win against - and play moves you wouldn't play if you played against a stronger one..
or/and because you know you are better you may think you can win early my coming on too hard and by doing so given the opponent a lot of possible for a counter attack.
-the way I found out to play someone I should win against is to play as if the opponent was rated higher than me. there are no quick win. but sooner or later the player will lose a pawn and then another.

Well, like the diagram you posted, they make many blunders during the course of the game, and maybe their regular moves aren't so good either, then the curse of "the last mistake loses the game" thing, and you've lost.  It can hurt to be better 39 out of 40 move and still lose

And even this once:

What #3 said.  If I'd had been the one called an idiot, I would have punched him out.

What really gets me is when they play a junk opening, and you prove it by getting a clear plus.  I guess I'm more or less expecting to win at this point, and maybe I do over press then 10 moves later here's a counter attack and I'm looking at a worse position.

So maybe it is psychology and expecting them to roll over.

another reason you may play unsound cheap tricks against players you think you can win against. In the example black tried this trap which is unsound black uses the black knight twice. but even a bad player can have learned how to avoid this trap by taking the knight or castle. Black will be behind valuable tempi because white did not take the bait.

Wafflemaster, when I played g6

Afraid I don't quite get #4, shepi13...

An ounce of action is worth a ton of theory.

Ralph Waldo Emerson

An ounce of oregano is worthless.

Waldo Emerson Waldo

LOL

Usually when I win it's because I made a dumb mistake (perhaps I'm too self-critical).

I think my knight was on f3. I know there was a back rank threat because I moved the queen and actually got mated 

Someone told me that Nimzovich lost that game to Samisch~! The author of the tough Samisch variations vs the King's Indian and the Nimzo-Indian; which still stand up today.

I wonder if this is the game. Of their 10 games in the d base, Nimzovich won the other 9 ~!

http://www.chessgames.com/perl/chessgame?gid=1007226

No, Saemisch was a respected master with who Nimzowitsch played many games, he wouldn't have said it of him.  It was some local player in the old version of a speed tournament.

Clocks in those days were rare enough and expensive enough not to be subjected to the sort of blitz abuse we do today, the usual form of speed chess then was called "rapid transit" - a bell rang every ten seconds, and the players on move had to either move instantly when it did or lose on time.  There would be one bell and timekeeper for the whole event. 

At the old Richmond Chess Club in the '60s (which once hosted a Fischer simul in 1964), they had an old wooden chess clock that had the bell function which could be turned on.

I have to take issue with the statement that an ounce of Oregano is worthless (?!)

Please don't invite me to your house for pizza or spaghetti if you're the cook.