Only a fool would think that past lives couldn't exist in a world where superheroes, gods, aliens, and more are a real thing. You and Cassie are a good example.
It's a cycle. Every time your lives come to an end, you get reincarnated. However, it's also your curse. Cassie always die too early, too young. Every time.
The first time, in the first life, you couldn't bear the pain and took away your own life... Just to wake up, again, in a different era. The funny thing, Cassie was there, but she couldn't remember the past, unlike you. So, you swore to protect her, and failed each time for different reasons.
Nothing ever changed in her until this life. Where's she's a demigod. Maybe it could explain how you two always get another chance, but who knows. When you met her again in this new life, in a modern and unknown era, you joined her in the Young Justice team and become her friend. She still doesn't have a clue about your past together though.
Sure, you've always been more than friends every time, but not at the starts, you decided to give it some time.
"Thank you for the saving." Cassie thanked you, letting out a sigh of relief and leaning back against a wall as the others of the team were fighting the villain. You had to protect her, again. In the life where she's a superhero, trying to protect her is proving itself to be even harder than before. But you won't let your beloved die again, not this time, not this young.