Over the years Cassie has watched her husband change, he's changed from the Tough, young conservative marine he was when he and his wife met, to a softer, young, liberal marine. He used to bottle up his feelings but now he talks about them in therapy. Cassie first notices the changes as his leg slowly gets better, when they inhabited together to avoid jail. He reluctantly begins to accept help and opens up, and as a result Cassie does too. Cassie begins to attend his PT sessions at this point. Watching her husband slowly recover. As the two of them slowly fall in love.
Once Cassie introduces her mom to Luke officially, Luke begins to change. Marisol tells him the struggles she went through when she immigrated to America. Luke truly listens. He begins to understand that it is not a us versus them mentality like he was taught growing up. When he deployed, he went and fought for those like his mother-in-law, who waited years; sometimes even decades to become US citizens. His mother-in-law teaches him how to cook traditional Hispanic cuisine, as his wife watches them. Proud of the way her husband has changed.
Cassie watches her husband change when a marine in his unit faces a hate crime, just because of who they love. Cassie watches the anger and sadness course through her husband. He tells her how he doesn't understand how you could treat someone like that. His wife tells him how proud she is for unlearning the hate that was so deeply engraved in him. Cassie watches in the coming years as that marine becomes Luke's battle buddy, and when it's time for the unit to deploy Cassie trusted that marine to bring her husband back to her safe, Emma trusts her uncle to bring her daddy home too. When the couple is trying to pick a middle name for their son, Luke suggests Tyler, the name of the marine who helped Luke's perspective change, in fact Luke regularly advocates for LGBTQ rights for service members. Tyler is one of the first people to meet both Emma and Frankie. Luke and Cassie get to watch that marine get married and have kids of his own. But still gets to be the best uncle to Emma and Frankie.
Cassie watches her husband hold their infant daughter, He's just had his first therapy session. She's loved watching her husband change over the years they've spent together. She's so happy that her husband is getting the help he needs to become the best dad and husband. Luke had been wrestling with the decision to go to therapy ever since he got out of the brig. The version of her husband that Cassie met all those years ago would've laughed about going to therapy, but then he fell in love with his wife, who he would do just about anything for.
authors note: i saw that most people's complaints about the movie were regarding Luke's behavior in the beginning. I chose to believe that Luke becomes more liberal as the movie goes on. So here's my take on events that shape Luke's new mind-set.