I'm not really sure why we're talking about companions falling in love with the Doctor like it's a regular thing. As far as I can tell, the only ones who definitely loved the Doctor in That Way were Rose, Martha and Yaz, and in the case of Yaz that's only been officially acknowledged within the show itself in the most recent season and the specials. The only other ones that come close would be Clara, though that was more a mutual attraction between her and the Eleventh Doctor that went away when he regenerated and Jack, though that seemed more an extension of his playful flirting with everyone. Maybe Sarah Jane, though that was more a retcon done after the fact in School Reunion. I guess there's River, depending on whether you count her as a proper companion or not.
Clara definitely while he was still Eleven, and for a time at least clearly Amy did as well.
Is it easier to identify companions who definitely didn't at least fancy the Doctor (in the modern era at least) Mickey, Donna, Rory, Bill, Ryan. Graham and Dan. (sure I've forgotten someone!)
It wouldn't surprise me if Nardole had a thing for the Doctor

Re classic companions. Jo Grant and Romana II could probably be added into the equation. I think there's clearly the possibility of romance in both cases, and it may be a stretch but at times you can feel something between Five and Nyssa, especially in Terminus.
The adverse reaction that certain parts of Who fandom have to the idea of romance between the Doctor and their Companions doesn't make any sense to me.
Also, River Song is officially considered a Companion.
River is definitely a companion. River definitely loves the Doctor. And the Doctor definitely loves River (despite her thinking otherwise). I have no issue with the Doctor loving River. Irrespective of Alex Kingston being older than someone like Billie or Jenna or Karen, the character of River is clearly far older than Rose or Martha etc. That said I have no idea how old River is though the consensus seems to be at least 200. I mean she spends 24 years with the Doctor at the Singing Towers Of Darillium on their last date alone and doesn't seem to age much. Irrespective, River feels far more of a contemporary of the Doctor than most companions.
This isn't just a problem I have with Who. A 900 year old man and a 19 year old girl is just as creepy as a 16 year old high school student and a 270 year old vampire!