Most people believe you will be in turbulence only when flying in bad weather, for example; storms, rain, or snow. That is incorrect: when the sun gets so hot on the ground, it causes the air to rise resulting in turbulence. As the air rises; it cools and forms, so pilots then have to look for smooth air above the clouds. When convection stops after sunset, it is much smoother but there are other forms of turbulence.
Obstructions to wind flow, like papers blowing around in circles between buildings, will also cause turbulence
When you fly over mountains, especially on the Eastern side of the Rockies, you will notice this kind of turbulence.
In large aircrafts, high altitudes near jet streams, winds occur that are different in speed and direction, or both.
Although turbulence is not dangerous, it would not be the truth to say it is not dangerous. It does make the controller's job easier when the aircrafts fly at predictable paths rather than flying as if they are going to fly into each other. There are some people who have gotten air sick when the airplane moves erratically. Straight and level flights make things more comfortable. When aircraft flies through turbulence, it rocks its wings, dips, and bobs because the air is moving every way.
There is no physical law that says airplanes have to fly in a straight line at a constant altitude and constant airspeed. Airplanes can even fly backwards for a short time and be safe, according to the article: "Basic Principles of Aircraft Flight."
So even though level flight may be preferred, if an airplane enters turbulent air, there is usually no real safety issue. According to FAA regulations,aircrafts are designed to withstand for more stress than occurs in normal flight, including ordinary turbulence. But, turbulent air in severe thunder storms can be so powerful that it can rip an airplane to piece, but that is only when a pilot would fly right into the middle of the biggest and meanest thunderstorms there are; but no competent pilot would ever do that deliberately.
There are, however; some small general aviation airplanes that have gotten destroyed in thunderstorms because the pilot was flying in the clouds and not having on-board radar to distinguish a thunderstorm from surrounding clouds. In error, they flow into a big thunder cloud.
Commercial Aviation Dispatchers who will plan flights will route them away from thunderstorms. Sometimes flights are even cancelled because of thunderstorms. They will also require a curse change to avoid bad weather, so if your flight is delayed or cancelled because of weather. it is recommended that you be grateful rather than angry.
Guidetopsychology.com/airfacts.htm#4 - "Basic Principles of Aircraft Flight," Raymond Lloyud Richmond, Ph.D.