From Muppet Wiki

Kermit is walking home from a day of fishing when his friends Fozzie and Gonzo notice that Kermit's wet feet make a funny sound when he walks. They laugh and joke about it all the way home, not noticing that Kermit feels embarrassed and ashamed.

Kermit obsesses over the incident all night, and by the morning, he's realized that his so-called "friends" probably despise everything about him. Kermit withdraws from his friends, and becomes introverted and suspicious. It never occurs to him to talk to anybody about it, or to put on a pair of shoes.