Thoughout this third season of Doctor Who, although I enjoyed many of the episodes and Martha was starting to grow on me as a character, there was one thing that absolutely drove me crazy. That was the way Martha pined after the doctor from day one, long before he'd shown himself to be deserving of it. With Rose it was different. She initially signed on with the Doctor not because of who he was, but because of where he could take her and what he could show her, and as they spent time together, their friendship grew stronger until they became inseparable. Martha, on the other hand, although she enjoys and participates fully in the adventures, seemed to initially be signing on so that she could be near the Doctor, and I just didn't see what was motivating that. But now I think I do. I was rewatching the beginning of the season because I had missed what this year's Bad Wolf/Torchwood equivalent was and now that it seemed to be coming to a head, I wanted to go back and find it. So I was watching the first episode and in the beginning Martha is kind of unsure of the Doctor, but thought he might know what he was doing, which was a sensible position given the evidence. Then, in order to save the world and for entirely non-romantic reasons (which he stresses), the Doctor kisses her, whereupon she gets all melty. Now with most people this wouldn't make sense, but since, as we all know, the Doctor is made of magic, I can see how being kissed by him might cause a person to fall immediately and completely in love with him and want to follow him to the end of the universe. It's still a cheap plot device that they could have done without, but it's better than no motivation at all.