This is actually sort of an odd question, but not for the reasons you might think. It's not an unreasonable question, and it's not like the answer is inherently unsatisfying or complicated. Actually, the answer is incredibly simple to the extent that absolutely everyone can understand it. What makes the question odd is that despite the simplicity of the answer, it's still asked constantly, and people still have problems solving it.

We get fat because we eat too much and exercise too little.

It genuinely doesn't get any more complex than that, although you can look into it. Fat serves a distinct purpose, it's the most efficient way our bodies have found to store excess energy. If you eat 3,000 calories and only 2,000 are used in the day, your body will turn the remaining 1,000 into fat, since it allows the energy to be stored in the long-term without having to exert the body (like building muscle). For every 3,500 extra calories in your body one pound of body mass will be created to store them, and this adds up over time.

Now a new question comes up. That 1 pound of mass doesn't have to be fat, it could be muscle! So why are we creating fat specifically? This is where things get a little more complicated, and we have to begin moving beyond calories into actual nutritional information.

One major point is the importance of eating protein. Where calories cultivate body weight in general, protein is responsible for muscle growth and retention. Since muscle is a less efficient source of energy storage, your body will burn muscle before fat if there isn't enough protein to sustain it. This is why people on hard core diets often find themselves losing as much, if not more, muscle than they do fat. People who exercise regularly may well find they never manage to tone up, with their calories going towards building fat and burning muscle.

So, that initial answer is technically correct. We do get fat because we build up an excess of energy in our bodies. If you have a deficiency of energy, weight will eventually be burnt off to make up the difference. But it's important to remember that while this will work 100% of the time, you have to play close attention to your diet in order to make sure it's not encouraging fat over muscle, whatever your weight.