Why does Milo decide to put the tollbooth together?

Of course, if you've ever gotten a surprise package, you can imagine how puzzled and excited Milo was; and if you've never gotten one, pay close attention, because someday you might.
"I don't think it's my birthday," he puzzled, "and Christmas must be months away, and I haven't been outstandingly good, or even good at all." (He had to admit this even to himself.) "Most probably I won't like it anyway, but since I don't know where it came from, I can't possibly send it back." He thought about it for quite a while and then opened the envelope, but just to be polite.

to be polite and curious