For a spy, love takes Natasha completely by surprise. It sneaks up on her. Insinuating itself into her psyche before she even has a chance to put her shields up. To be quite honest, she wasn't aware she needed shields. Surely her time in the Red Room had totally and completely wiped out her capacity to love? Being partnered with Clint and gaining him as family, and by extension, Laura and the kids, was such a pleasant surprise for her. She cherished them and thought that that was all her heart could take.
Then…the helicarrier. Then...Bruce's eyes disappearing into the Hulk's. Then…the Hulk, hand poised, ready to end her. Then…the realisation that, in more ways than she thought, Bruce was just like her. Trapped into something he had no control over…in a body that was a weapon... just like hers. And so…kinship sparked in her heart.