Cassie tries to sneak a look into his book. Bart slams it shut before she can. Cassie teases him and says he has a diary among other things. Kid Flash hums in agreement. Impulse yells and attempts to tell her that it's not true.
These two voices in his mind struggling to control his actions. This is why Bart stays quiet most of the time.
It would be better than unleashing those two on the world.