She says she wants to travel with him forever.
Humans don't get forever; they get seventy or eighty years, strung together like Christmas lights, and then they die. The Doctor's a thousand years old, give or take, but he's never had that sort of life. He spends his years scattershot across history, a week here, a few hours there. He hung around Earth for a while back in the 1970s, and he thinks that if he added up all the time he's spent in the twentieth century he might get some kind of significant number, but it's not the same. Humans think of time as linear, but to the Time Lords it's just another force to be toyed with.
Rose says "forever" and she means every day that ever was or will be, but the Doctor knows it doesn't work like that. Time travel's tricky, and the Doctor's only interested in the exciting bits. They'll never visit 2248 (an exceptionally boring year) but he's been to August 11th, 1966 five times on three different planets. He'll never see every day, no matter how long he travels, but he's still got forever at his disposal.
He takes her back in time to watch bits of rocks coalesce into a lump that will become her planet. It's not the beginning, but it is a beginning, and he thinks she understands. He's not the same man who took her to watch it die, but her hand in his feels the same as before.
He'd give Rose her seventy years if he could, but she made him promise not to leave her behind, and he means to keep that promise. He can't give her forever, but he can give her the very beginning and end of the world. He hopes it will be enough.