After the war in Lanka, Rama was crowned king of Ayodhya and Bharata became the crown prince.
Rama had offered to make Lakshmana the crown prince but he refused, saying Bharata is elder than him, and more deserving of the title.
Rama hearing this was very pleased and said " O lakshmana in this birth you served me so well and did your duties as a younger brother I will do the same in next birth as your younger brother".
Thus in next birth Rama becomes Krishna and lakshmana becomes Balarama elder brother to krishna.

Lakshmana is the one who leaves Sita in the forests near sage Valmiki's ashram after she expressed her desire to leave the kingdom.
Lakshmana remains loyal to his brother and fights against Rama's sons Lava and Kusha later.
After the war in Lanka, Rama was crowned king of Ayodhya and Bharata became the crown prince. Rama had offered to make Lakshmana the crown prince but he refused, saying Bharata is elder than him, and more deserving of the title. Rama hearing this was very pleased and said " O lakshmana in this birth you served me so well and did your duties as a younger brother I will do the same in next birth as your younger brother". Thus in next birth Rama becomes Krishna and lakshmana becomes Balarama elder brother to krishna.
Lakshmana is the one who leaves Sita in the forests near sage Valmiki's ashram after she expressed her desire to leave the kingdom. Lakshmana remains loyal to his brother and fights against Rama's sons Lava and Kusha later.
Lakshmana