1. Cardiovascular Diseases - When the human body is relatively inactive for a very long time, the heart becomes weaker and less efficient than it should normally be, and is more susceptible to life-threatening diseases that can cause heart attack.
2. Obesity - Like gasoline is to an average car, food is fuel to human beings. It is our energy source. Unfortunately, if people maintain (or even increase) their eating habits from their youth, yet become more and more sedentary as they grow older, they use less and less energy as they simultaneously take in more and more. The body then stores this excess enhergy as fat,and obesity occurs in no time. Obese people are even more susceptible to cardiovascualr diseases, so it's really no laughing matter. Read more about managing the body's energy consumption on my article about Energy Balance.
Backache - If you don't maintain your muscles, they atrophy, and become weaker and weaker over time. Thus, weak back muscles commonly cannot maintain proper posture for long periods of time, whether standing or sitting. This causes the myriad of people you see on the street with a hunchbacked posture, which is incredibly bad in the long term. Placing the spinal column outside of its natural curve for long periods of time puts additional pressure on areas of the body that weren't meant to maintain that sort of stress, thus resulting in back pain. And you don't want that.
4. Chronic Tiredness - Again, if your body's inactive, your muscles atrophy. Not only that, but the rest of your physiological systems weaken over time as well. Your body's not as efficient as its supposed to be, causing you to not function as well as you should be. This results in the feeling of chronic tiredness. People wake up tired, they go through the day tired, and when they go to bed, they can't sleep for some reason, despite the fact that they're still tired. So they turn on late night TV until they doze off, and wake up tired once again. Why? Their internal systems aren't function very efficiently.
5. Anxiety - This is actually directly related to the above. Studies have shown that people who have been inactive for long periods of time often suffer from constant anxiety. When your body's not functioning very effectively and very efficiently, you just don't feel right throughout the day, which can actually cause you mental stress, resulting in constant anxiety.
6. Depression - Depression is another observed mental side effect of constant inactivity that studies have confirmed throughout the years. Combined with such things as obesity, back pain, and so on, it's really no wonder that very inactive people are usually depressed.
Still want to remain inactive? Hopefully not.