It's completely irrational to be angry at Adric, but every time he smiles, Tegan wants to put her fist through his face. The Doctor is no better; she can't even begin to understand this alien who could die and be reborn as a new man.
Nyssa is an alien too, but at least they have grief in common. Though her aunt's death is nothing compared to the losses the other woman has suffered, it's a matter of degree, not kind. It makes the other woman seem comprehensible, almost human.
In this strange world, it's all Tegan has to cling to.