I've wanted to make this post for a while and I finally have enough evidence that I can. Whenever Clara says 'I love you' to Danny she doesn't really seem to mean it.
1. The first time she says it in an episode where they breeze through the entire relationship so it feels rushed. On top of that it's in an episode where she lied the entire time and she said it out of spite. I am convinced she only said it to annoy the Doctor.
2. The second time she says it, it's actually meant for the Doctor. This has been confirmed by Jenna Coleman.
3. The third time is over the phone and it feels like Clara only said it so that she could hang up on Danny without having him worry about her. Again it was rushed and more robotic than emotional. Something that's commonplace so you just do it because you feel you have to, not because you mean it.
4. Finally in this last episode, the one time I could have believed Clara because those words now belong to Danny and she goes to such lengths to see him again. And then at the end of the episode she says 'I love you means nothing right now' which really speaks volumes. And she hangs up on Danny soon after he repeats it, effectively giving up all hope.
Compared to the Doctor who could never get those words out and yet does everything he can just to see Clara happy. Even after she betrays him, it doesn't matter, he'll still help.
And Clara to the Doctor, she's only said it once when she was sure he was looking away. But she favours the Doctor over Danny-wanting his praise of Danny's phone call, the Doctor was her first call instead of the children's parents.
In this case, actions do speak louder than words. Someone else stated it but it needs to be said again:
Clara and Danny say 'I love you' a lot. But Clara and the Doctor actually act like they love each other.