Why do some people get costochondritis and others don't?

Usually viral illnes: Costochondritis, or tietze's syndrome, is an inflammation of the costochondral junction along the anterior chestwall adjacent to the sternum. It is usually caused by a viral illness, and like all viral illnesses who gets it is hard to predict. No genetic predisposition has been suggested. A strong immune system is probably the best defense, i.e., good rest, good nutrition, etc.

You've been lucky: Inflammation of the front of the rib cage where it meets your chest bone-sternum. It has connections formed of cartilage. The cause can be many things. One of course is potentially viral, also indirectly a bacterial infection causing a patient to cough so much the movement of their ribs is irritated. Immune deficiency diseases as sarcoid; other causes as trauma and curvature of spine incr chance.