Chapter 9 of Katherine Paterson's young reader's novel Bridge to Terabithia is set during early spring, and the weather has turned very foul. Both Leslie and Jesse have school off for the week of Easter holiday, but it rains constantly, interfering with their plans to spend the whole week in Terabithia. It's due to the fact that the inclement weather keeps them from enjoying Terabithia to the extent they want to that Leslie decides their kingdom is cursed.
One day, though it is raining heavily, Leslie decides they should venture to Terabithia regardless and dons rain gear. Jesse is shocked to see how high the creek's water has risen and feels it is unsafe to cross over, but Leslie does it regardless and Jesse follows. They keep returning to Terabithia on the next two rainy days as well. On a Wednesday, as they sit in the castle stronghold they had built, it begins raining so hard that their shack leaks. It is at this moment that Leslie turns to Jesse and says, "Methinks some evil being has put a curse on our beloved kingdom." She also suggests they venture to the sacred grove and ask the Spirits how to "combat" the curse. As she phrases it, "For of a truth I perceive that this is no ordinary rain that is falling upon our kingdom."
Hence, as we can see from this chapter, the only reason why Leslie plays with the notion that their kingdom is cursed is because it is raining so much during the week that the rain is preventing them from enjoying their imaginary kingdom to the fullest.