Why do chart patterns happen?

Can someone explain the reasoning or logic behind why they happen?

The subject interests me but I cannot find any satisfactory answers.

Well, triangles and flags are pretty easily explained.
It either has something to do with indecision, accumulation or distribution.

Let's say the market can't decide which economy is better, the EU or USA. In that case you got a minor movement which gets smaller and smaller - that is then indecision and then you will see a triangle.

For accumulation: the big guys are positioning themselves before the trend starts or during a break of the trend, they gotta build market positions of several hundred million dollars or billions dollars. To make that happen, they have to do that over several days. That means that they either create a triangle or flag. Due to their large size, price will go against the position. So if they are building positions with the trend, a flag is created. Cause each time the place a position in the market, it goes against the position, so the market retraces.

For distribution: Well, and when they are done riding the trend, you will normally also see some ranging patterns, in which they load off their positions. If there is a run-aways market against the trend, it normally means that they try to immediately go out of their positions due to some fundamental shifts. That creates large volatility.

At least that's my interpretation of why these patterns are created.

Thanks for the good info - it makes sense when it is explained properly.

If you have read "A Random walk down wall street" , you will realize that these patterns can happen in most of charts of communities , future , stocks or something else even a Random face of coins chart .

If a tree falls in a forest, and there is no one around to hear it, does it still make a sound?

Each pattern has its own story. I am sure that I have heard most of the stories and I have probably forgotten most of them.

Double (sometimes triple) tops and bottoms are easy to understand. A market has a good run up and becomes overbought or expensive. Selling and profit taking starts to happen. Then people believe that the prices are cheap again and start buying driving the price up again. When it reaches the old level or a bit higher, selling takes over again. This time is different as the buyers abandon hope of another rebound and join selling it down to lower levels. So you end up with a double top. Double bottoms are the opposite of course.

The patterns occur because humans are able to devise and recognize patterns.

Patterns occur because your brain thinks they occur. I dont believe they provide any basis to make trading decisions.