So, his main reason for leaving within the film is not so much on Tauriel not returning his feelings, but rather of her banishment from their home. So when Legolas informs his father he will not be returning to Mirkwood at the end of the film, I do believe that it is in co-relation to his previous statement. Legolas: You may tell my father that if there is no place for Tauriel, there is no place for me. Within the actual film, Battle of the Five Armies, we are given a scene where Legolas tells his father's messenger that if Tauriel is banished, he will not return. Although Tauriel isn't canon to Tolkien's work, it is possible that within Peter Jackson's films, Legolas's return to Mirkwood could possibly have been associated with Tauriel returning to Thranduil's realm after the events of The Hobbit.

What we do know is that he does return to Mirkwood as his father sends him to the Council of Elrond during the events of Fellowship of the Ring. However, Legolas had departed before Thranduil's meeting with Tauriel and probably never heard of it until later. And maybe she doesn’t go back to being head of guard, maybe she’s been softened and wouldn’t resume her old position, but in a way I feel like maybe she would be even more impassioned to protect what she loves, to protect her home, to protect what’s important to her, but maybe she’ll do it now with more compassion and a little bit of softness. She’s not head of the Elven army - that is Prince Legolas and King Thranduil - but she’s the head of the Elven Guard, and they protect their realm. It sounds so boring, but ultimately, she has a job. You know, it’s going to sound very mundane, but I think she goes back to Greenwood - which would hopefully now be Greenwood slowly but surely it would evolve out of Mirkwood and return to Greenwood - and she goes back to work.

This was brought up and answered by the actress Evangeline Lilly herself in an interview about the character's fate after The Battle of the Five Armies: I personally think that it was implied that Thranduil lifted Tauriel's banishment after the events of The Battle of the Five Armies after their encounter at the end of the film. However, the main three reasons that I have found that resulted in Legolas's departure from Mirkwood in BOTFA are these: Tauriel's Banishment I think this answer is more of a combination of various interpretations.

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