1. Jack thinks Donna is amazing. A much better person than he could ever hope to be. He wants her to stay that way and doesn't want to corrupt her too much.
2. He likes her a lot. And doesn't want it to be just the one time thing. But he doesn't want to pressure her, either. He probably will bring it up sometime, but not right away.
3. He probably should talk about how much the Doctor has screwed him up with her, but he doesn't like to talk about it with anyone, especially people he cares about. Doesn't want her to know the horrible things he did and doesn't want her to feel sorry for him.
4. Jack was actually a bit upset about the whole Jenny thing. He can't blame anyone for thinking it was his fault, but for once it actually wasn't. And having Donna pissed at him.... well part of him did like getting smacked, but that's just Jack.