To is to do more than just speak; it is to declare something with earnest, solemn intensity. It’s a word for moments when "I think so" isn't enough, and only a formal, emphatic "I am certain" will do.
You don’t asseverate that it might rain; you asseverate your innocence in a court of law or your undying loyalty to a cause. 2. Etymology: From "Earnest" Roots asseverate
Authors use asseverate when they want a character to sound particularly pompous, desperate, or profoundly certain. It’s a favorite for: To is to do more than just speak;