For better or worse, there’s no definitive law in publishing about whether or not to italicize direct thoughts or set them roman. Words into Type references using italics for thoughts, while The Chicago Manual of Style, 15th ed. leans more toward roman type, with neither tome issuing a true edict on the matter.
YA novels commonly sport direct thoughts because that’s a handy visual cue for younger readers: Not in this lifetime, she thought. (And with italics in play, you can often omit the “she thought” part, which is a nice bonus. Most of the time, the fewer words, the better.) Adult books are more likely than YA to use roman type for direct thoughts: Not in this lifetime, she thought. Ultimately, this is a matter of “house style.” That is, each publishing house (sometimes even each imprint within a house) picks one mode over the other and instructs all their copyeditors to mark up manuscripts accordingly. The good news is that without an authoritative decree in place, you’re free to indulge your preference. As long as you apply it consistently, you’re good to go. When the time comes for your editor to transmit your final manuscript to the copyeditor, you can request your mode of choice for the final book. If “house style” calls for the other mode, so be it. It’s not an issue worth battling over.