Certainly, there are many reasons for your development of a low body image. Each person has different causes. This article lists some possible causes which are right to many people.
You have been brainwashed by society's norms. Some people are more sensitive than others to the messages about how you "should" look that media bombards them with. You may be one of these people. What is "the right way to look" is different in different societies. If you feel that your body doesn't match your society's norm, and you are very concerned about fitting in and "being normal", you can easily get a low body image.
You have been influenced by your family. Do the others in your family have a low body image? In that case you may have "inherited" it from them. Is everyone else in your family better-looking than you? That's another reason to why you may feel ugly. Have you received negative comments about the way you look from your family? Again, this may have led you to start to dislike your appearance.
You have been influenced by your friends. If all of your friends also think negatively about the way they look, it would be surprising if you did not think in this negative way as well. Similarly, if you socialise with many people whom you consider to be better-looking than yourself, this would be a reason to why you feel the way you do. Perhaps your friends consider appearance very important and talk a lot about make-up, clothes, diets, exercise etc. If so you may find it difficult to reach their high standards.
You have a low self-esteem in general. If you have a generally low self-esteem you will feel negatively about most things about yourself, not just your appearance. It is difficult to feel beautiful, sexy and handsome if you deep down feel like you're not a valuable person.
You have been encouraged to dislike your body or discouraged from liking it. Think about what comments you have received from others. Did you ever say anything positive about your body and were told off for boasting? Did others tell you that "good people" are modest? Did you ever say anything bad about your body and others agreed with you or encouraged you by not replying at all?
You compare yourself with how you think you "should" look like. Are you comparing your present body with how you looked when you were young? Are you comparing your present body with how your parents looked when they were your age? Are you comparing your present body with how you think that you should look considering your healthy lifestyle?