Weight adds wear and tear to your joints because there is more pressure, your heart and lungs also have to work harder, there can be a buildup of fats which clog your arteries leading you to cardiovascular failure such as heart attack or stroke, possibly amputations etc. Obese people also are much less likely to fare well with many, many other health conditions and diseases in general

Obesity - especially abdominal obesity - is part of a broad cluster of conditions collectively known as metabolic syndrome. This syndrome includes, among other things:

• Diabetes
• Cardiovascular disease
• High blood pressure
• High cholesterol (or more properly, high LDL and low HDL, the "bad" and "good" cholesterols respectively)
• High blood triglycerides (fats)

These things are all interrelated in complex ways.

For example, fat cells are related to insulin, in the sense that insulin stops fat cells from releasing their stored fat. Insulin, in turn, is related to high blood sugar, in that high blood sugar stimulates the release of insulin. But high insulin levels for extended periods can result in insulin resistance and hence diabetes, which causes high blood sugar to stick around and fat cells to improperly release fats into the bloodstream. Those floating fats can build up in blood vessels, causing high blood pressure and ultimately heart disease. And the poor circulation from that disease can combine with the poor circulation caused by diabetes to...

...yeah, like I said, complicated.

In part, becoming very obese is as much a symptom of underlying breakdowns in the way the body regulates energy intake as it is a cause of those breakdowns. It's all part of a bunch of feedback loops where which is cause and which is effect becomes kind of unimportant, because they're all causing and reinforcing one another.

Its easier to understand if you ever see those cross sections of cadavers, but when you gain body fat, its not just pushing out, its also pushing in, and it adds weight to you. This puts pressure on your organs and your muscles and bones(lungs and heart included) to keep up and support you. Because obesity is often caused by poor eating habits and/or lack of exercise, and your body is now storing a large excess of fat, it tends to go hand in hand with high cholesterol(which can lead to blood clots and heart disease) and high blood sugar(which can lead to diabetes).

There are a lot of reasons, if we can separate them generally into two categories:

1. What the actual fat does to the body
2. How people become obese

For the first - The hormones released by fat can cause many problems, the fat itself can literally squeeze and crush the internal organs (that is what happens with OSA), the weight ruins your joints, etc..

The second - No one becomes morbidly obese by eating carrots and apples. You need to consistently be eating really shit food for a long time in high quantities to become truly obese (theoretically it is just calories in and out but realistically you can’t actually overeat on cabbage). These types of foods have a great impact on hormones, your blood vessels, your gut microbiome, and so many other factors that basically negatively impact almost every system in your body. This paired with being less mobile (also due to step one) can have a dramatic impact on your cardiovascular system.

To really explain everything would take literally hours, but basically these two categories are responsible.

To explain it like you are five well there are different types of fat cells Visceral fat produces more toxic substances than subcutaneous fat, so it is more dangerous. Even in thin people, having visceral fat carries a range of health risks. It is a type of fat that wraps around your internal organs. Unfortunately being obese means you most likely have that type of fat as well.