Why doesn't Ralph ride the motorcycle though he puts on his helmet?

In the beginning of the book, in chapter 2, Ralph, the mouse, does not ride the motorcycle, because he does not know how to start it. He worries that maybe it's only a toy and it doesn't really run at all. He pushes it with his feet but ends up rolling off the table and into a wastebasket.
In chapter 4, the boy, Keith, finds Ralph and the motorcycle in the wastebasket and lets Ralph out. Keith tells Ralph that in order to start the motorcycle, he must make the noise. Ralph is exhilarated with his new activity.
The next day, Keith goes with his family on the picnic. Meanwhile, Ralph ends up riding the motorcycle even though Keith told him not to. He gets stuck in a laundry hamper, and he loses the motorcycle. The boy brings back a helmet, half a ping pong ball, and Ralph sadly admits he lost the motorcycle.
Eventually, in chapter 13, the bellboy returns the motorcycle. Keith generously allows Ralph to ride it once again. Ralph puts on the helmet and rides briefly around the room. He wants to wave, but he knows a safe driver keeps both hands on the wheel. Ralph has grown, if not in height, in wisdom. He has learned from his mistakes and will refrain from joyriding and being careless.