I think the problem with Clara is that she doesn't really have a story behind her and she has no motive at all. They have taken no time out to develop her character.
Rose: she was a girl who lived in South East London who was fed up of her mundane life which explained why she ended up traveling with the doctor. Her character continued to develop throughout her adventures with the doctor.
Martha: She couldn't handle all the demands of her life from her over baring mum, her sagar daddy dad, his gold digging gf and from her training as a nurse. She met the doctor and he took her on an adventure and she ended up developing feeling for him.
Donna: she originally rejected his offer to travel with the doctor and regretted it since she then spent ages looking for him and she found him she was already packed and ready to go
Amy: since being a little girl she's waited for this raggedy man in a blue box to take her on an adventure she grew up to be a crazy bitch who stanned for someone everyone thought was in her imagination except he ended up being real and he took her away.
Clara: she was a babysitter having internet problems and she called the wrong number.....
There was more character development for the companions in the classic series and that's in like the 60s