Why did Heidi forget Peter and his goats?

Heidi's been cooped up indoors taking care of Ursula, a deaf old lady. She's surrounded by all this beautiful Alpine scenery, and yet Heidi's unable to stray from the confines of the hearth. But one morning, Heidi's grandfather tells her that she can accompany Peter and his goats to the mountain. The little girl doesn't need a second invitation, and she jumps for joy. But first, says Heidi's grandfather, she must wash and make herself tidy. So she pops back inside the hut and quickly splashes some water on her face until she positively gleams with cleanliness.
When she emerges from the hut, Heidi is greeted with a truly joyous scene of natural beauty:

During the night the wind had blown away all the clouds; the dark blue sky was spreading overhead, and in its midst was the bright sun shining down on the green slopes of the mountain, where the flowers opened their little blue and yellow cups, and looked up to him smiling.

Heidi starts running around gaily, gamboling through the meadows like a spring lamb. So enraptured is she by the bounteous joys of the mountain that she briefly forgets all about Peter and his goats.