I think the issue was that RTD tried to make them too similar and into this perfect couple, but it was just too much to watch. Her chemistry with the the Doctor had some real balance.

I think when you have an older Doctor, you can have those type of hyper young companions. But when you have a hyper young Doctor, you need more "mature companion". Take the 4th Doctor for example. He was a lot like Tennant's Doctor, so his companions (even Romana II), tended to be more mature and serious around things,

And yeah having them treat their love interests like shit didn't help.