I got a different answer from my friend for the above question and thought of sharing with you all. When sita first saw rama, she immediately liked him and prayed to god that if she marries rama then she will do pooja for goddess for 14 years in forest. That's why they went to forest and because of kaikayi's wish.
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i want to share something regarding this that ram knew ravana would come and take sita so he sent sita to agnidev one day before . and the sita who was with ravana was the image of sita.so its was the hindu epics for the purpose to destroy the demons from the earth.so everything was preplanned .
See Rama is the first wife son of dhsarathan.
Barath is the 3rd wife son of dhsarathan.
3rd wife name is Kaikeyi. In earlier she got 3 boon from her husband(dhsarathan). she want to make her son as king . So she used one boon that sita and rama has to spend 14yrs of life in forest.
When i was kid i watched like that rohit. Am sorry .Even i asked my mom she also told 3 boon.If possible some one please surf net and tell me clearly . Am lazy guy.