The boy thinks in terms of the worse-case scenario — it’s not so much jumping to conclusions as already being AT the conclusion.
example #1: Last week he managed to trip down the stairs and land with his toe curled under him. The boy decided he must have broken it. Admittedly, his foot was not looking pretty. But in retrospect, considering he’s all healed up one week later, the facebook photo & announcement “I broke my big toe” might have been a tad premature.
example #2: This morning the boy inexplicably threw up. And then sat on the couch and said: “I think I must have cancer.”