"But she loved him, as well." That was the other thing that bothered him. Rick loved Ilsa and Ilsa loved Rick. It confused him, if they both loved one another, why they simply gave that up. "As to why she let him, I don't know." Castiel's frown only grew at this, showing how perturbed he was at the conversation and not being sure of what the characters within the film were thinking when they made this decision or that. "You say she would have been trapped and he knew that, as did she. She didn't wish to be trapped, although she loved him." He didn't understand the full scope of love, how one would sacrifice their happiness for the sake of another person. He saw how the Winchesters laid themselves on the line for one another and knew it wasn't simply because they thought they only had each other. It was for a multitude of reasons and one of those happened to be love.
"Rick made the choice without Isla," he pointed out after a moment, because it was a decision that she was forced into, thanks to the arrangements being sprung on her at the very last moment. She hadn't the amount of time needed to fight back, given the limited amount of time that was available to them. "And as I have learned, it is not always best to make such choices for others. It does not end well." He could only think of the occasions he took on the burden he felt he must carry and either failed or, how with the souls from Purgatory and the Leviathan that came with it, made an ill informed choice that could have been avoided if he'd had access to that help much, much sooner.