You remember the scene from AoU when Natasha tells Bruce about the initiation procedure in the Red Room? They both feel such strong emotions, that it is impossible he could just walk out after that. So, I am trying to fill in this gap.
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Bruce feels terror clutching at his chest when he hears what they had done to her, what they forced her to undergo. Her beautiful face betrays how broken she feels. The revelation was impossible for her to say and it was almost impossible for him to comprehend. And that last phrase of hers:
"So, who is the real monster here?"
It feels like the last nail. Her pain hits him hard and he can feel his heart replying, aching for her.
After her last words Natasha is focusing on the floor. She is trying to take her breathing under control, to tuck away those emotions again. She has to appear strong! Strangely enough, the feelings do not listen to her command; she feels as if it was a great weight of regret settling down on her.
Bruce registers the stinging sensation in his eyes, the tears for her. He feels incredibly sad and angered at the same time, angered towards those monsters who did that to her.
Most of all it is hard to see Natasha like this: sad, broken, lonely and rejected. His tactics in life always involves staying as far possible from people as the current stretch allows. He does not come closer, he does not do friends, let alone lovers, because he is too dangerous - a true and a perfect monster. But right now, right at this moment, not doing feels like hurting her even more. Before all his precaution gear kicks in, Bruce finds himself moving.
Natasha senses the shift in his stance - he is going, he is moving, leaving the room, for good, for now he knows and he is sure to run. She can't help but take a glimpse of him, so she is lifting her head just in time to register that he has been moving towards her. He does not pause. Nor does he stop at an arm's length. In mere moments, Natasha finds Bruce right in front of her her: a torn look on his face, his shirt open. Next thing she knows: he is wrapping his arms around her. No hesitance, she registers. Bruce pulls Natasha closer. She feels so petite, so small, and so fragile. He wants to keep pressing her to him, to give her comfort and reassurance and... something else he is not ready to name. Natasha sneaks her arms under his shirt, around his waist and up his back. He feels so big now when she does not wear the protective suite, or the high heels. She can feel the muscles in his back stretching as he is pulling her further, deeper in his embrace. All through her body she is feeling him, his warmth. It is a soothing feeling. She does not feel alone any more.
"Natasha, I am here, I am right here" - she now hears that Bruce is repeating those words, whispering them in her ear. He is so strong - she thinks. She lets go and just succumbs to the moment. He is saying something else, he is so very close, she loses track of time.
Eventually, they hear Laura calling everyone for dinner. Natasha tears her head from Bruce's chest, she doesn't let go though. Neither does he. She is looking up, seeing nothing but his big chocolate brown eyes.
Bruce smiles, just a little tug on his lips. Brushing a strand of hair from her face he kisses her temple, then her forehead, his lips lingering there. She hears him speak, still whispering, as if he is afraid to ruin what they just have built.
"Natasha, I can't give you more. Not right now" - His voice is gentle and frightened. She gets it - not with Laura and the kids in the house, he is afraid, he is not sure of his own reaction. To hell with it, she is not sure of her own reaction. Gently holding her chin with his forefinger, Bruce touches her lips with his own, brief and tender: the ghost of a kiss, a promise.
Laura keeps calling, they realize. Natasha breaks the silence:
"I need to hop in the shower; I'll be ready in 5".
He releases her then, smiling, brushing down his palm along her hand.
'I'll tell Laura we will be in 5".
"We?" - She smirks.
He is wearing that dorky look of uncertainty she absolutely loves again.
"I.. hm… uhm… I'll wait on you right here" -He motions around the room.
Natasha disappears into the bathroom. Sitting on the bed, deep in thought, Bruce touches his lips: the feeling of her permanently engraved in his memory.