1. "I'm thin..."

2. "... so people are fat because they choose to be."

Anecdotes do not equal data. Stories about you and your family are not enough to form an entire theory of weight management for a whole species. Many scientists have done actual research in this field so that you don't need to draw conclusions based on your personal experience.

Your body isn't the same as every body. There are genetic components to weight, as well as extensive medical and medication factors that either cause weight gain or make weight loss difficult. I've already listed twenty in this thread, information about which is available in a fast, easy Google search.

People don't understand weight because they choose not to.