There was something primitive in the way she kissed him, something hungry, something needy and yet not, something that would devour him whole. And he is surprised; surprised at her kissing him when she has just told him she is getting married. It feels good, he has to admit, it feels more than good and a foggy haze almost clouds his judgement enough for him to want to kiss her back.

If you could only see the beast you've made of me
I held it in but now it seems you've set it running free
Screaming in the dark, I howl when we're apart
Drag my teeth across your chest to taste your beating heart

And he does, it is only a fraction of a second, but the doctor kisses Amy back like she is kissing him, full of urgency, desire, hungry. Because she is right, it has been a while and there is nothing quite more in that fraction of a second than the want and need to draw his fingers over her skin, to let her hear his two rapidly beating hearts.

My fingers claw your skin, try to tear my way in
You are the moon that breaks the night for which I have to howl
My fingers claw your skin, try to tear my way in
You are the moon that breaks the night for which I have to

But he stops, a little voice stops him and he pushes her back.

But it isn't the "I am 907 years old, I don't get old, you do, I change," or the "This will never work," or even "You're getting married in the morning."

They are all excuses.

He can't cope with having his heart broken again and with her famous words of "Nothing long term," something in him dropped down inside. Not his heart but expectation. He wants her to kiss him, he wants her to devour him, but not out of some silly lust, desire to take and have and discard.

The doctor wants more than that. He has a lifetime of bad experiences; he can't go through another one, not even for Amy. He can imagine, kissing her, running his hands through her silky red hair, he can imagine biting down to her skin, to devour her heart and soul. He wants her and he gave too much of that away when she said she wanted to go home, when she showed him his engagement ring and when he said "It would never work."

Now there's no holding back, I'm making to attack
My blood is singing with your voice, I want to pour it out
The saints can't help me now, the ropes have been unbound
I hunt for you with bloodied feet across the hallow'ed ground

Amy wants fun right now, she wants nothing more. To taste, to devour, to have the ecstasy right now and live her life before making the biggest mistake of her life. That wasn't kissing him right now but that she would be marrying another. The doctor has felt fleeting glances of jealousy before, but this is the first time he has felt the sickening punch of it, enough to send him reeling at the thought of her with another.

Till death do them part?

Another reason he can't, death will part them and he can't live with that knowledge that he could love her and then one day she would be gone. But this was the pain, the pain behind the knowledge; Amy kissed him because she wanted to in the heat of the moment. The doctor kissed back because of the feelings he kept so close in his soul.

Like some child possessed, the beast howls in my veins
I want to find you, tear out all of your tenderness

So many times it has been unrequited love from another; he never thought the shoe would be on the other foot. So that's why it must stop. Inside a part of him is howling, the primitive part of him that wants to continue, while also howling at the curse of the timelords and their ability to love.

Be careful of the curse that falls on young lovers
Starts so soft and sweet and turns them to hunters

If only she knew, the huntress, yet if only she know that he was the hunter, lovers that are cursed to be start so sweet and yet turn out sour.

The fabric of your flesh, pure as a wedding dress
Until I wrap myself inside your arms I cannot rest
The saints can't help me now, the ropes have been unbound
I hunt for you with bloodied feet across the hallow'ed ground