Don’t you think there’s a difference between saying it in Doomsday, and saying it in Journey’s End? A difference that makes it heroic not to say it when he is finally given the chance once more.

If the Doctor says, “I love you” as he’s saying goodbye in Doomsday - both knowing that it would be impossible to bridge the gap, it is what she longs to here, what she believes and the most honest of goodbyes. It would have given her some degree of closure I’m sure, which is why I think he goes to say it. There’s something to be said for the lack of distinct choice in this scene.

If the Doctor says it in Journey’s End, as they are physically together, it is different. It is the choice that they can’t go back off in the TARDIS together. Time apart has allowed the Doctor to rationalise the future of their would-be relationship. She can’t stay with him forever. And here he is, able to give her another Doctor, just like him. One that will grow old with her. If he was to say “I love you”, it would be an irresistible invitation back into the TARDIS. I think he knows he can’t say it. I think he knows it has to come from the other Doctor, and only the other Doctor.

He doesn’t refuse her it to be cruel. He does love her.

But sometimes there’s knowing when not to say.