One of the key fundamental areas of study in the marketing arena, including public relations, is consumer behavior. In the context of PR, you can substitute "consumer" for "publics".
In a very broad sense, it's important to understand the interplay between emotion and logic when attempting to influence attitudes and behaviors. From a purely marketing standpoint, this is focused on getting target customers to make a positive purchase decision. Of course, in PR this is only part of the equation.
Think about the last major purchase decision that you made. Was it based on emotion or logic or some combination of the two? What factors contribute to the relative weight of emotion vs. logic when making a choice between alternative products or services? In other words, is emotion more important for some types of products than others (and vice versa)?