7 Answers

Am I supposed to take your word for it?  You haven't given any dimensions. How about you edit your question and honestly give me your height and weight, and throw in your age, too. 
If you truly are overweight or obese (more "politically correct" than fat),  we can figure out how you got that way. 
This is apparently your "debut" on akaQA.  Your questions have a better chance of being answered if you are not so vague. You are setting yourself up for relentless ridicule by asking an open-ended question like "Why am I fat?" .  Don't say I didn't warn you.

Any number of reasons from food allergies to medicines to just being lazy.   You may not be any of these.  A person inherits their shape from an ancestor.  Tendencies also.  My grandmother was 5'2" and about 300 pounds.  She worked all the time and wasn't lazy.  Her mother was slim.  My other grandmother was so skinny, she could slip thru a key hole,  Guess who I look like????? 

No, I am not 300 pounds but I am not thin either!  I have to be careful all the time.  I wish everyone could be average!

ITS WHAT YOU EAT

What is "fat"? I'm only10-15 pounds overweight and I think of myself as fat but, it's not a health problem yet...  Be a little more specific...

For most ... it's the quantity that a person eats at meals on top of lack of exercise. Some have what they call " over active thyroid " . If you are too heavy and feel like you don't have any energy.. then seek a doctors advice. Most doctors will tell you to eat 6 to 8 times during the day, but small quantities of fruits, non fat products like cheese, milk, bran, veggies, and so on. It worked for me.

The biggest contributor to being overweight is over eating,a Doctor or Dietician can set you a healthy eating plan so you can loose the unwanted weight.

rather than focusing on the problem focus on the solution..