The Doctor can do pretty much whatever he likes, this much he knows. He could materialize into the Louvre in Paris, sonic open the glass case covering the Mona Lisa, grab the painting, and be gone before anyone knew a thing.
But he knows that, even if he did steal the Mona Lisa, he wouldn't keep her, because he's got something better to wake up to every day.
He's not rich, that much is true, but it would be easy enough to pop back in time and buy a couple winning lottery tickets, or perhaps some stocks in Google. He could buy the Taj Mahal, if that's what it took to keep his Rose, and if she wished him not to, he'd never even visit it at all.
Or perhaps she might want him to go back and count the number of bricks in the Great Wall of China as they're being laid. And that's just what he would do, and then the next day, he'd do it again. She's more than worth it.
He could rearrange every star in the sky to spell out her name, or he could drag the moon down to her, but he knows it doesn't matter, because she shines brighter than anything else out there.
And she blinds him; every light in every city on every world might be glowing strong, but he's still blind without her there to hold his hand, because she's the only one who opens up his eyes.
Yeah, the Doctor could steal the Mona Lisa, but he thinks he'll save her guards the heart attacks.
Because he's got something better.
Inspired by "Mona Lisa" by The Summer Set.
I do not own the characters, Mona Lisa, or The Summer Set.