If you look in the owner's manual of a car with an automatic transmission, you might find that the manufacturer says changing your automatic transmission fluid is not necessary. While this can be true, it's still a good idea for drivers to do so if they want to prevent damage to their transmission.
The reason why many manufacturers say changing your transmission fluid is not needed is due to the way the fluid is designed. Most transmission fluid is designed to operate up to 175 degrees Fahrenheit. If the heat inside your transmission never rose above this temperature, than the fluid would indeed last for nearly the entire life of the vehicle, or at least 100,000 miles. However, this is really only true of normal driving conditions.
Driving up steep inclines, towing a heavy load, sitting in a lot of stop-and-go traffic, driving at a high speed when its hot out or trying to get your car out of the mud can all cause the heat of your transmission to build up. And once the temperature begins to rise above 175, the fluid starts to burn up.
As a general rule, an increase of 20 degrees can result in the fluid's life being cut by half. So if the temperature in your transmission got up to 195 degrees, your fluid would likely only last about 50,000 miles. If you go even higher than that, you could quickly begin wearing out the fluid and doing damage to your transmission.
According to the Automatic Transmission Rebuilders Association, 90% of all transmission failures are caused by overheating. Transmissions are an expensive repair that most amateur mechanics will not be able to complete on their own. Thus it's a smart idea to pay close attention to your transmission fluid, especially if you notice anything different with your shifting.
Checking the transmission fluid is fortunately not a major hassle. There's a dipstick attached to the transmission that will allow you to do so. Ideally, you want to see a clear magenta or red color in your transmission fluid. You'll likely be able to tell if the fluid has been burning up, as it will give off a smell and appear to be brown or black in color. If you notice this when checking the fluid, be sure to change it out right away.