When Miranda approached Dom in the bar one night after shooting, he didn't realise at first that she was flirting with him. He'd had quite a few drinks by then, and alcohol always slowed down his thought processes.
When it finally penetrated his brain what was going on, he wasn't sure what to do. He thought she might be equally drunk, and didn't want to do anything either or both of them risked regretting later.
But then, her own feelings on the matter were perfectly clear. When she moved closer still, Dom put his arms around her and kissed her.