Perhaps a bit overdone of a title, but it has to be said that thinking is not always in your best interest. Your inner dialogue has an uncanny ability to keep you from acting appropriately due to hesitation, overanalysis, fixation, or distraction. Have you ever gotten excited when doing something well (particularly with beginner’s luck) and then had it all fall apart? It is likely that you thought about it too hard.
For example, let's take the node that spawned this title: How to Juggle. Fox Hunte's writeup suggests that you need to stop thinking. Why is this? It can't be muscle memory yet, as you've hardly practiced the action. However, your body (and I am including all the subconscious/non-conscious bits of your mind here) has picked up the rhythm — and if you keep thinking about the actions, the mind is likely to get confused and attempt to correct when no correction is needed. Suddenly, you've knocked everything out of order.
Muscle memory, the concept of autonomous action, is another example worth examining. What do you think about when driving to work? For me, I rarely have to think "correct slightly to remain in lane" to myself — it happens automatically. If I had to micromanage everything that was going on, it would be impossible to attain any speed while driving. For an even worse example, what about walking? How many of us think about each movement individually? Could you break it down into each action? In Tai Chi, and in many martial arts, one is taught precisely how to move. It's hard work, but done with the idea that eventually the action will occur without thought. If one does not move perfectly in a fight, instability and weaknesses may appear. Of course, muscle memory can also lock you into robot mode, where you act as an automaton within the world, so as in all things, balance is key.
When working without thought, the Centipede's Dilemma also comes into play. If you're doing something from intuition, you might find it hard to analyze how it works. This is related to the concept in chaos magic of lust for result and therefore the drive in chaos magic to utilize vacuity. The idea is that if you are thinking too hard about doing something, you will sabotage it. Those familiar with certain parts of Buddhist philosophy or Joseph Campbell's ideas on heroic temptation may note a certain resonance with this idea. It even makes an appearance in Star Wars, when Luke uses "the Force" to destroy the Death Star.
More interestingly, there is the state known as hack mode, where the rest of the world fades away and one is hyper-focused on the task at hand. Often such states allow faster and better work because the mind is submerged in the task at hand. With the clarity brought on by a lack of inner dialogue, progress is fast and easy.
There are other ways in which thought can cause problems. Fixating on negative events, for example. It is good to figure out why you got knocked off your horse, but it's rather foolish to wallow in the mud you were tossed in. One of the prime rules given in Dale Carnegie's How to Stop Worrying and Start Living is to keep going on with life rather than falling prey to worry. Action is often preferable to examining events in excess detail.
Similarly, it is also necessary to allow oneself to listen to intuition. I have had some bad experiences with ignoring intuition about a situation. However, particularly in Western culture, rationality is played up and intuition is played down, producing a rational inner voice. This inner voice will happily talk you out of any hunches you may get. This wouldn’t be so bad if intuition and the subconscious did not have a history of coming up with the right answer.
When one learns how to think, one should also learn how to not-think, how to let intuition and the subconscious make one’s life easier. There are times for analysis and times to let the rest of you work its own magic.