Completely agree.
Seeing a kid going through a terminal illness has to been extremely hard, even more when the family is not there for the kid. So I do think it's natural for the doctor to want to give more attention/support. Combine that with the fact that he also never felt wanted in his family, I think it was unavoidable for there to be a strong bond.
In Sarina case she knew she didn't have must time and her life was that hospital room. He probably gave her more attention then anyone else in her life, so I don't think unatrual for her to lable those feelings as love. There was no other feelings to comapre it to.
In the present Aqua cares deeply for Rubi, even when he wasn't sure she was Sarina. In his previous life his grandmother made him feel out of place and lonely. So on a new life I believe he would be attached and overprotective with whoever was his family.
Rubi was not obsessed with Aqua, she only became so when she found out he was the sensei. She is basing her romantic interest on a idealised dream version of who sensei was, and not on how she actually has sees Aqua. At this moment she has Sensei lens when she looks at Aqua.
A romantic relationship just seams so far off. I think the lesson for them is family love, something that both lack in previous life. And even Ai was only able to tell them she love them in her last moments.
Am I the only one who thinks that there's no way the author is actually going to the incest route? Idk I just think Aqua/Sensei always considered Ruby/Sarina as a little sister figure and Ruby/Sarina is just hung up on her past admiration and she's going to eventually move on from it, could be wrong but (ϋ)/♩ I believe Aqua/Sensei is a responsible adult.