He is his step father.Mannu is a teenager and teenagers react differently.There is a chance of him all of a sudden hating his step father without any reason.People when get hurt they forget all the good things done with them by the person who somehow has hurt him.Mannu might start thinking Jagya has no right on him even if he asks him to do a normal thing.Why taking chances!
All these things could have been avoided.He was just 10 years old when Jagya wanted to reveal it.If that time it would have been revealed then Mannu would have reacted but then Jagya would have get the time to show how much he loves him.And as he was a baby his reaction would have been different.
Lastly when I saw Ganga saying those harsh words to Jagya just because he was trying to make her understand that truth should be revealed and then after many years that same truth will bring sadness in his and his son's life though he tried to avoid it then I find Ganga responsible for all this.Whatever Mannu's reaction will be towards Ganga he will not get any opportunity to taunt her as stepmother but in case of Jagya there is a slight chance and its all because of Ganga thinking that why telling him!
The worst thing will be if Mannu will get to know from others that he is not Jagya's son.Didn't expect such irrational behaviour from Ganga.
