Can't speak from personal experience, but I would imagine "why" is at the top of the list along with "is there something more I could have done". Dealing with the inevitable sadness/guilt that will be left behind is no doubt one of the challenges to deciding a plan to CTB that extends beyond just choosing a method.
I can't imagine a better answer to OP's questions than this reply right here.
To add a few additional questions that no one has said yet loved ones might also think,
"How could I have gotten to know Person X better? Could we have spent more time together?"
"Was leaving painful? What was it like and did they feel all alone?"
"How can I possibly move on without person X in my life? What would person X would have wanted with all the things they left behind?"
"What did person X really think of me? Was there anything they wanted from me? What did they dream of and what kind of things brought them pain? What secrets may person X have had or kept to themselves?"
"Did person X really want this or did they make a big mistake? Did they really consider the consequences? How could they do something like this? How could they leave us like this?"
