Because that close the other person's eyeballs are giant, like bulbous bug orbs with big sweeping eyelashes that loom like batting rakes given the proximal perspective. It's like being attacked by a monster or something.
Or they think the other person is ugly and don't want to see.
Or, like tasting a fine wine or sumptuous dish, you just shut down other senses so you can focus on the tactile and taste experience.