Sokka's absolute failure at catching a fish is just the beginning of a terrible series of events. There's the whole thing with the platypus bear and that leads her to Aunt Wu. Watching Katara get all googily eyed over the fortune teller worries her.
"Who's next?" The fortune teller asks.
Sokka sighs as she gets up. "Okay, let's get this over with."
"Your future is full of struggle and anguish. Most of it, self-inflicted." Aunt Wu said dryly.
Sokka just laughs bitterly. "I could've told you that." She goes back to looking for more of the puffs she had been eating before. She's sure that Momo took them.
After they all get their fortunes they wander around the little town. Sokka's still sure that the fortune telling is a big hoax, but Katara must have heard something she liked because she's convinced Aunt Wu is a prophet.
"You're just saying that because you're going to make yourself unhappy." Katara says in an all knowing voice.
"Well," Sokka says irritated, "Even if that is how my life ends up, it's gonna be because of what I do, not because of what some fakey fortune teller says."
When Sokka learns that the fortune teller is going so far as to say that the clouds can tell her if the volcano is going to erupt, she can't help but want to get out of that town as soon as possible.
Sokka gets dragged along with Aang to get some flower and pretends to be irritated about it, but in all honesty sort of wants one of the flowers for herself. When she gets there, she absolutely horrified to learn that she was right. Aunt Wu was wrong and the volcano is about to erupt.
Sokka is so happy to see her plan work out. When someone undercuts her work by saying that Aunt Wu predicted the village would be safe and it was, Sokka wants to curse. Instead she takes some deep breathes and just keeps reminding herself that her plan worked.