Tegan's departure hits you like a ton of bricks. When most companions leave, it's all "I've fallen in love and I want to stay with him" or "I have to stay. These people need me."
Tegan was the one companion who finally reached her limit on how many people she could watch be killed while traveling with the Doctor. It's one thing you never expect to see, even though it's one of the most realistic reactions a companion could have.
