they think that saying "I love you" will make all of their problems disappear, will make all the hard decisions disappear, will make all of the pain and suffering and consequences disappear, but it doesn't. It's just words.
Saying, "I love you" does not stop you from hurting the person you're saying them to. It does not grant you forgiveness or righteousness. It does not mean you are doing the right thing. It doesn't make you the hero.
Saying, "I love you" doesn't undo or make right any of the things you've done wrong.
Saying, "I love you" doesn't mean you get a happy ending now. It doesn't mean your problems are gone. It doesn't fix anything.
It can't solve anything. It doesn't magically fix relationships, because it has nothing to do with what caused the problem in the first place.
The Doctor telling Charley, "I love you" does not make up for what he did.
Charley telling the Doctor, "I love you" does not fix what has gone wrong between them because whether or not they're in love with each other has no bearing on the issue.
It's not a matter of "okay, if I say it back then it means that things are okay between us after all." because no, it doesn't. Him saying it in the first place did not help anything. It was just an excuse.
He made his decision. And saying "I love you" did not change that decision or the consequences of it. It did nothing to help. He did nothing to help. He made his choice and his choice led to nothing but pain for everyone around him.
The Doctor telling Charley, "I love you" does not negate the cruelty of what he did when he said it. It doesn't make what he did, to Charley or to the TARDIS, okay.
Charley telling the Doctor, "I love you" does not mean that she wasn't hurt and it's all perfectly fine now and not a problem. She refuses to ever acknowledge when she's really hurt or in pain or scared, and this is just a continuation of that, only now instead of pretending it didn't happen, she's pretending that if she says it back, it makes it okay.
Except it doesn't, and it's not okay. It never will be.