Bilbo had been having such a great number of debates with himself over how he should act and react and say and not say over the course of the journey that, on the morning of Gandalf's first visit, he has no plan for dealing with the wizard.
He'd barely even remembered to get out onto the bench.
"I'm looking for someone to share in an adventure."
Bilbo feels his jaw go slack, and he knows he looks ridiculous, but that's besides the point, because laughter is suddenly bubbling up, and it's all he can do to force it down.
Maybe he's going into hysterics.
It's just... Gandalf is well and truly mad, isn't he? To march right into Hobbitton, and try to persuade the first hobbit he sees to go on adventure? And to tell dwarves that they would be getting a highly skilled burglar, when in fact, all they would be getting is a gentlehobbit whose mother knew Gandalf? Bloody insane.
Gandalf is going off about something to do with Belladonna and Tooks and being late to dinner (sounds rather ridiculous), when Bilbo abruptly stands up, and says, "Come in for tea, and we'll see about this adventure of yours."
The wizard is startled out of his lecture, mouth closing-though only momentarily.
He gives Bilbo a measuring look, and Bilbo is sharply reminded of the thought haunting him all week.
His actions have repercussions, and the more he changes, the less he will be able to predict. Indeed, his actions can have quite unforseen consequences, and can greatly affect the future. Look at what happened with the Ring.
He is suddenly worried about what this might change.
"Or-or-or any other day really, I'm not sure I'm in a decision-making mood, nor an adventurous one. You might have better luck over the hill or across the water!" He scampers (my, mit's been a long while since he'd been able to move like this) up the path, and shuts the door firmly behind him. Bilbo doesn't pause beside the door, instead striding deeper into Bag End and (hopefully) leaving Gandalf to his carving.
Later, as Bilbo slips out of the smial to make a run to the markplace, he notices the blue mark, glittering softly in the light. He ignores the tears springing to his eyes, and casually allows his fingers to caress the mark.
"Bless you, Gandalf," he whispers, "May all the Valar smile on you for this."