It had been that kiss.
That one kiss he gave her the moment before he ran to go with his counterpart. It had been in that moment that she knew. That one second when she realised that there was no way he was going to be coming home with them.
That kiss she didn't return.
It was that kiss that woke Rose Tyler to the cold hard fact that she didn't love Mickey anymore.
And the fact that he didn't i need/i her.
That kiss, which was such a cruel parody to the one she had given him when she had left for the Doctor's life.
Now he was leaving for his own.
She hadn't closed her eyes to the soft lips on hers.
She hadn't parted her lips.
And she knew now that he had been trying; searching for a reason, right then, just one reason for him to stay.
It could have just been a quick peck on the lips. But it had stretched just beyond that.
Long enough to react.
To give him a reason.
Something worth living the lie for.
And she failed to give him that.
And now he was gone.
And when he returns, he'll be dragging her back with him.