I think their they built a friendship and in the mean time they found feelings for eachother. Now they just want to share their feelings but can't do to rule 12, "Never date a co worker". Their ment fro eachother and they both know it.On episode "Aliyah" it shows that tony loves Ziva after killing Rivkin for Ziva, because he never loved her and on episode "Bury your dead" when Tony's car blows up it shows Ziva loves him.