Jon begins his discovery of how being in a relationship can make one as profoundly unhappy as not having a girlfriend. Today (and potentially this story arc) is about the increasing phenomenon of being forced into things you don't want to do to spend time with someone you like. This factors into all intimate human relationships, but perhaps it is one of the primary differences between a girlfriend and a platonic friend: they expect more attention and indulgence. Not that Liz makes particular demands of Jon, and seems to be simply inviting him to the lecture if he feels like going; the telling moment is Jon's eagerness regarding all things Liz Wilson. He's beginning to lose both a sense of self and in the process of overdosing on Liz, risking the mystique of a new relationship by spending an increasing amount of time in activities that bore him. At some point the average person would begin associating the activity partner with the experience of boredom and disinterest, but luckily for Jon, his own hobbies lean toward the dull and/or nonexistent.
Garfield, it is good to know, is such a studied liar and so accustomed to zoning out while dullards babble at him, that he immediately knows how to handle the situation, and advises Jon thus. Garfield has also been so long and so deeply mired in boredom that he believes sincerity itself can be faked. One may wonder when Garfield, who tends to wear his heart on his sleeve and speak bluntly, would have cause to present a facade of interest; the answer is: every day that he bothers to get out of bed and face the world.