The ending to La La Land is so perfect it's one of my favorite endings of all time. I've seen a lot of people label Mia and Sebastians relationship as "right person, wrong time" but I disagree, I believe their relationship was right person, RIGHT time. Mia and Sebastian had a domino effect on each other's lives that led them to success, making their dreams a reality. 

   1. Mia always had a passion and love for plays and had been wanting to write one but didn't have the confidence to follow through. Sebastian challenged this and pushed her into actually taking the leap.
   2. Sebastian initially dismissed Keith's offer to become the keys in his band. However, overhearing Mia on the phone with her mom about how Sebastian plans on opening a jazz club and how he DOES have a plan but nothing solid and no stable income, Seb decides to take up Keiths offer and meet with the band so he could have a stable job. This marks the start of his busy schedule and eventual tour that becames a stressor in their relationship.
   3. With Sebastian on tour and Mia finally working on her one woman play, they became busy and unable to see each other often, leading to their relationship being stretched thin. Sebastian decides to surprise Mia with a romantic homemade dinner, which starts off smooth and sweet until Sebastian floats the idea of Mia coming with him to Boise. With her play being only 2 weeks away she tries to explain that she will be too busy rehearsing and preparing, but he fails to understand why she can't just rehearse there. Mia takes this as an opportunity to confront him about how he doesn't even like the music he's making and how this wasn't what he wanted for himself. This causes a heated argument resulting in Mia leaving the apartment and increasing the strain in their relationship caused by distance and lack of communication.
   4. On the opening night of Mias play, Keith reminds Sebastian about a photoshoot for the band that is planned at the same time. Seb goes to the photoshoot and misses Mias play. After Mia takes her bow, the lights come on and we find out that only a few people showed up. The disappointment on her face is apparent (specifically when she realizes Seb isn't there) and she goes backstage to sit down where she then listens to 2 men laugh at her and belittle her hard work. When she goes outside, her emotions running high, she's met with Seb apologizing and pleading for forgiveness, but it was too late. All the failure, embarrassment, hurt, and feeling of betrayal Mia has comes to a breaking point and she says that it's over. Their relationship AND her acting career. Then Mia gets in her car and goes home (her childhood home).
   5. Now we see Seb sulking on his bed, upset from the breakup, when he gets a call from a casting director trying to reach Mia, meaning she most likely turned her phone off to keep him from contacting her. This does not work as he is able to find her house and tell her about the audition. She immediately turns it down having no confidence of being casted, but he doesn't take no for an answer, showing up the next day, taking her to the audition where she gets the part that becomes her breakout role.
(If they HADNT broken up, Mia wouldn't have turned her phone off for privacy, which means she would be the one receiving that call about the audition which she would have likely turned down because she couldn't handle more embarrassment and rejection and had no confidence in succeeding. She would have missed out on her big break.)
   6. Seeing Mias success in the acting world and actually having the money, Sebastian finally decides to listen to Mia about the band not being what he wanted for himself and returns back to his ACTUAL dream of opening a jazz club, even using the logo and name she created for him. (We know Seb was planning on staying with the band long-term because of his argument with Mia).

The montage at the end of what Mia and Seb's life could be together was (in my opinion) an idealistic fantasy, a "what if?" that would follow both of them for the rest of their lives.

This is my own personal opinion/interpretation. Feel free to agree or disagree. Everyone has unique thoughts and feelings.