Tegan's swansong is probably the most emotional departures ever, besides the departures of Jamie and Zoe. She is sick and tired of all the death and destruction that occurs in Resurrection of the Daleks, and there was quite a lot of it. It was an incredibly dark story for her to leave on. The reaction from the Doctor is actually quite heart-wrenching, and chasing after her makes it even more painful to watch. This episode is definitely a tear-jerker and it's quite obvious to see the Doctor and Tegan did have a close friendship, no matter how much they tended to bat heads with one another. She had been with the Doctor since he had regenerated, so of course the Doctor may take it a bit harder than his other companions. Especially when she's actually leaving from things she has seen before. Warriors of the Deep had tons of deaths in it, and it didn't effect her as much as it did the Doctor. So it makes people wonder what makes the Daleks and Davros so different from what Tegan had seen in previous adventures. Tegan was originally to have stayed up until end of that Season, but they decided to quickly write her out. Maybe it was to make room for Peri,but it could be for any sorts of reasons.