There is actually being fat and there is thinking that you are fat and they are not always the same thing. The easiest way of measuring whether you are actually fat is by knowing your height, your weight, your age, and your sex (we hope) and then finding out your Body Mass Index on this site.
Some of us may think we are fat when we are not like people who are anorexic, and others may not think they are not fat when they are!
There are may reasons why people don't want to be fat...
"People would love me more." People will love you anyhow - it is you that matters so you don't have to be as thin as Kate Moss as fat as a sumo wrestler to be loved.
"I would find it easier to get the boyfriend/girlfriend that I want." Fortunately some people like fatter people some people like thinner people but you do want to be more huggable than a balloon or a matchstick!
"Everyone would think that I look totally wonderful." Beauty is in the eye of the beholder.
"I would feel much, much better about myself." Worrying about being too fat or too thin can become an obsession for a person. If you are within the normal range you won't have to worry.
"I would be fit, and be fitter." There is no one correct weight for everybody, but people who are overweight tend to be less fit and are more likely to have health problems. Being too thin because you are not eating properly can also cause health problems.
"If I am fat now it is more likely that I will be fat as a grown up." This is true so it is best to get into health eating now, along with taking some exercise.