Her departure may be more like Tegan's, who just saw too much death and destruction.
It was especially hard for Tegan because some of those deaths were people she was friends with, not to mention what the Master did to her Auntie Vanessa. She witnessed Adric's death, plus the deaths of some of the people she'd become friends with. And although Nyssa didn't die, she did leave to go to a run-down hospital ship that had the same reputation a leper colony has.
Well, maybe there's another companion that has the same kind of intermittent relationship with the Doctor that Clara does -- living in their own home (maybe in a different era) and occasionally getting picked up by the Doctor for an adventure. Heck, he could have a whole slew of part-time companions who never meet. Although I have to wonder why he'd keep them from each other.
I kind of miss the days when the Doctor couldn't steer the TARDIS and going off with him meant you probably wouldn't see your home again for months or years, subjectively. The modern come-and-go approach sort of cheapens things. Being a companion should be something that sweeps you away and totally changes your life, not just an occasional break in a quotidian routine. I was hoping that Twelve's "Do you happen to know how to fly this thing?" upon his debut was a sign that we'd be going back to those days, but no such luck.
If the Doctor is "two-timing" his companions, it's probably for at least two reasons: First, because sometimes they don't get along (ie. Tegan and Turlough), and second, one pregnancy in the TARDIS was quite enough!
I agree; traveling with the Doctor should be an amazing once-in-a-lifetime opportunity, not something that annoys you because you're having a painfully awkward first date you just have to get back to.
It makes sense, though. Susan was younger than Romana, who was 140 in her first story. Susan could easily have been a few decades younger than Romana, yet older than Ian + Barbara.
He didn't leave Mel; Mel left him.
I know Sarah Jane was left behind because the actress was leaving, but later on there weren't any problems with taking Leela and Nyssa to Gallifrey. Sarah was cheated!
Companions normally leave on their own and the fifth Doctor was in 20th century England several times, if she had wanted to leave before she finally did she could've. I don't think the Doctor had any real problems with Tegan, but it was Mel he left on Iceworld with Glitz if that shows you how he felt about her.
That's actually incorrect. The end of
Time Flight was his first opportunity to dump her off and he did.
Strictly speaking,
that's incorrect.
Black Orchid took place in the 20th century. More than 50 years too early for Tegan, but still in the right century!