Jack Shephard had many appealing qualities. As a strong leader with an impressive medical background, Jack was a determined and resilient individual. These admirable qualities--along with his physical attractiveness--made many characters romantically interested in him both on and off of the Island.
Jack ultimately ended up with Kate and there were many factors that made them a strong couple. With that being said, there were plenty of other Lost characters who could've been a good fit for Jack. These are the other characters he could've been with and the reasons that Kate was the right choice for him.
Could Have Been With: Ana-Lucia
When Jack and Ana-Lucia first met in an airport bar, they immediately hit it off. Jack was still in the throes of grieving over his father's death, but Ana-Lucia managed to make him laugh and helped him see that there would be better days ahead.
When Jack learned that Ana-Lucia was the one responsible for accidentally shooting Shannon, he immediately changed his vengeful, angry tune and instead wanted to resolve the situation peacefully. He did his best to acclimate her into the larger group of Oceanic 815 survivors. They shared some flirtatious moments and their determined, passionate personalities allowed them to connect with each other.
Kate: Trust
When Jack was wounded in the pilot episode and needed someone to sew his wound, he placed his trust in Kate. She was a complete stranger to him, but he chose to trust her in his moment of need. He opened up about how he'd faced his worst fears in the past, showing that he was willing to entrust her with a story of his own insecurities and challenges, while also finding a way to build up her confidence and help her feel calmer.
Whenever it really counted, Jack knew he could trust Kate. Another example of this was that even though she'd never delivered a baby before, Jack was confident that she could deliver Claire's baby while he was busy trying to save Boone.
Could Have Been With: Libby
Jack and Libby never shared any romantic spark or flirtatious moments, but they could've been a good fit for each other. Through Jack's work as a spinal surgeon and Libby's career as a clinical psychologist, both of them dedicated their lives to helping other people through their intellect and unique skillsets. They continued to channel this dedication and selfless focus while living on the Island.
Libby pushed Hurley to become the best version of himself in a way that felt loving and positive. After a lifetime of feeling like he never lived up to his father's expectations and all the self-loathing he endured, Libby would've been a positive and healthy influence for Jack. They would be a good intellectual match and they shared many of the same values.
Kate: Teamwork
Since they could trust each other when it really mattered, Jack and Kate were able to work well together and make a good team. Both were fiercely determined individuals, but they also knew how to be a balanced team and play to each other's strengths.
Kate was an excellent tracker and incredibly resourceful while Jack had more medical and leadership skills. Through this kind of teamwork, they were able to accomplish things like defeating the Man in Black and finding and reviving Charlie after he and Claire were taken by Ethan.
Could Have Been With: Sarah
Jack "fixed" Sarah and repaired her crushed spinal column after a horrific car crash that seemed like it would leave her paralyzed for the rest of her life. Through the relationship they formed, Jack believed Sarah had also "fixed" him and they went on to be married, although the marriage ended in divorce.
Under different circumstances and if different choices were made, Jack and Sarah could've stayed happily married. If Jack had shown the same dedication in his marriage that he showed when "fixing" Sarah, things might've worked out differently.
Kate: Competitiveness
Couples usually benefit from a healthy sense of competition. Jack and Kate definitely shared a competitive spirit and always wanted to be the best.
They were competitive with each other, such as when they competed against each other in golf to prove who was the better golfer. It wasn't just about proving who was the best; they used a healthy sense of competition to flirt and further bond with each other.
Could Have Been With: Gabriela
Gabriela and Jack began to fall for each other as he worked tirelessly to save her father's life. Her father ultimately didn't survive the surgery, but she still felt drawn to Jack. While grieving over her father's passing, she kissed Jack and he kissed her back.
She still believed in him and wanted him to be her solace. They could've ended up together, but the shared goal that brought them together was gone, and without it, it's unlikely that their relationship would've survived.
Kate: Together On And Off Of The Island
Jack and Kate found ways to make their relationship work on the Island and in the "real world." They faced many challenges in both environments and they faced their fair share of ups and downs.
Regardless of the circumstances and obstacles they faced, Jack and Kate didn't let the environment they were in dictate the terms of their relationship. Instead, they found ways to adapt, like when Jack overcame his conflicted feelings about Kate raising his nephew Aaron, and Jack stepped outside of his comfort zone and became a father figure to Aaron. They adapted and created beautiful moments together both on and off the Island.
Could Have Been With: Juliet
Jack and Juliet developed a romantic relationship on the Island, but it didn't last. When Jack was a captive of the Others, Juliet was the only one to show him kindness and treat him humanely. Jack was often filled with self-loathing, but she helped him reconnect with some of the best parts of himself, and he did the same for her.
They also bonded over Ben's attempts to manipulate them and the hatred they felt toward him, along with their fierce desire to escape the Island. Nevertheless, Jack's heart belonged to Kate and Juliet knew it. If Kate had been out of the picture and if Jack hadn't escaped with the Oceanic Six while Juliet remained stranded on the Island, their romantic relationship might've survived, though.
Kate: Jack Needed Her To Move On
Jack was only able to move on in the flash-sideways because of Kate. When she took Jack's face into her hands, he experienced flashes of his life. He resisted his awakening, but she took him to the church, where he fully reawakened and remembered his life.
This allowed Jack to be reunited with the love of his life. Jack and Kate were meant to be together forever. They needed to remember the love they shared and be reunited before moving on with the rest of the group.