I am pretty sure that the dosh in Shiv's kundali is that he will die young. This is why he is having the nightmares where he is running from something (death) looking scared. Gauri's kundali must have something which will grant him that long life. That is why Nikhila was particular that Gauri marries him -to ensure Shiv's long life. But by granting him his life, her kundali will be weakened. So she may die in a year.
Shiv may not have wanted to marry Gauri because he hated her and loved Ipshi instead. But Nikhila would have told that for the sake of his long life and his desire to live with Ipshi he has to suffer the semblance of a marriage for a year by the end of which Gauri will die - giving him the long life and clearing the way for Ipshi who is also Nikhila's best friend daughter. So that is also convenient. Shiv may not have believed in the power of the kundali but agreed to the marriage to please his mother and father (who threatened to disown him and not give property). He thought that he can get Gauri to divorce him so that he will be free. But Gauri would'nt leave him.
The first incident in the forest where Gauri was shot was the turning point in his life. He started to believe in the kundali. So he was even more desperate to protect her. So he brought Shashank to tell the truth. But in the process he learnt that she loved him. So it became even more difficult to tell her the truth because he did not want to hurt her.
The fire was the last straw. He forgot all about the plan. All he wanted to do was to save her. So he admitted to her that she is in danger but refrained from telling her that if she leaves he will die. If he had told her that I'm sure Gauri would not have left.
This then is the irony of fate. Two souls interwined in one destiny. Gauri would be willing to sacrifice her life if it means saving Shiv's and Shiv has already shown he is willing to risk his life if it means it will save hers. If this is not love what is?
Ipshi or no Ipshi; Shashank or no Shashank - Shiv is for Gauri and Gauri is for Shiv.