They didn't talk about it because they didn't have to. Anyone within the distance of a mile could feel it radiating off the both of them and if you were lucky enough to be able to see it, then you were blessed. She heard, before she travelled with him, that in Greek Mythology it is said the Greek gods were bored, so they created humans. They were still bored so they created love. They were not bored anymore and created love for themselves. Then they created laughter, so they could bare it. She thought this was a fantasy for those who wanted there to be something more in the world. Well, she's seen that there's more, so much more and she's discovered that those gods had it absolutely spot on. Love is contagious. When you see it, you want it, when you've got it, it breaks your heart and the only cure is to laugh or cry. She had this with him and it was FANTASTIC.
She told him once, on a beach, on the worst day of her life. She didn't need to but he was saying goodbye and she'd seen how his mind worked. She would not be Sarah Jane, she would not be forgotten. How dare he walk into her life and impress her with a blue box and a sonic screwdriver and all that running and mean more to her than all the species in the universe and then forget her just because she wasn't there? And when he disappeared before he could even try to reply, she died a little inside.
Then he looked up and she was there, looking at him the way she always did. A hint of a laugh, with pinch of a smile and love and sadness all wrapped up in her big green eyes. He hadn't forgotten her and he looked at her like she was the most important thing in his universe. She had returned for him. She risked her life for him. And she wouldn't have it any other way, because it was BRILLIANT.