What Dharma forces Lakshman to leave his wife and accompany Lord Rama, yet Lord Rama takes Devi Sita with him?
Vanavasa was only given to Rama, then why Sita went with him?
Sita to Rama:
Valmiki Ramayana 2.27.4
भर्तुर्भाग्यं तु भार्यैका प्राप्नोति पुरुषर्षभ।
अतश्चैवाहमादिष्टा वने वस्तव्यमित्यपि।।
A wife alone, O best of men shares the destiny of her husband. I am also therefore, ordered to dwell in the forest. (a command to you is a command to me).
Valmiki Ramayana 2.29.8
अथ चापि महाप्राज्ञ ब्राह्मणानां मया श्रुतम्।
पुरा पितृगृहे सत्यं वस्तव्यं किल मे वने।।
O highly sagacious Rama I had also heard in my father's house long ago, from brahmins prophesing that some day I shall have certainly to dwell in the forest.
Why Laxamna went in forest with Rama?
Sumitra to Laxamna:
Valmiki Ramayana 2.40.6
व्यसनी वा समृद्धो वा गतिरेष तवानघ।
एष लोके सतां धर्मो यज्ज्येष्ठवशगो भवेत्।।
Rama is your refuge in times of adversity or prosperity, O sinless one, to be obedient to the eldest (brother) is the duty (Dharma) of virtuous men in this world.
Valmiki Ramayana 2.40.9
रामं दशरथं विद्धि मां विद्धि जनकात्मजाम्। अयोध्यामटवीं विध्दि गच्छ तात यथासुखम्।।
Regard Rama as Dasaratha, Sita as me and the forest as Ayodhya. My child, go.
So Laxamna followed his Dharma (duty).
Now question arises Why Urmila didn't go with Laxamna in forest?
However, Valmiki Ramayana doesn't provide a satisfactory answer to this question but you may read this article which is written based on other versions of Ramayana or you can follow the below question in this site: