His nod is one of grave understanding and gratitude. "Thank you then, milady, for saving me from myself. To think, I would have continued my days, meditating, trying to teach patience to the perpetually-impatient, and amassing knowledge and wisdom such that I would rival the greatest Jedi Masters. Truly, it's a horrible fate, and I am in your debt for rescuing me."

It is now that Obi-Wan is struck with the thought he's been unable to capture, that niggling doubt and worry he's been suppressing since arriving here, alone with Padmé. Now that he is relaxed and enjoying her company, it is as clear as the lake they're overlooking.

The thought is this:
He could easily find himself falling in love with her.

After all, this conversation proves just how easily they can get along as friends, how well they enjoy each other's company. He isn't sure if it's a true premonition through the Force, or merely his trained talent for seeing patterns and outcomes, but he can see it happening easily, as smooth as glass.

And he can't let it.

Padmé has a long career as a senator ahead of her, and Obi-Wan is her protector. More, he is a Jedi Knight: despite being assigned to one person for the rest of her life, he cannot allow his emotions to cloud his judgment. Friendship is one thing, but love, a real relationship, even if she were amenable, goes against everything he's been taught.

Obi-Wan's smile slips slightly, but he turns to lean on the railing beside her, looking out over the horizon, hoping she will assume his mind has simply wandered to something else.

The truth is, currently he's not sure he could think of anything other than her.
It is the opposite of what he needs to be contemplating.