This episode really introduces the new series to the reality of the companion. Up until now, this generation of new fans had only known one companion, but she is soon to leave the show and others will come in. Rose is now the most important person in the Doctor’s life, but, like Rose, Sarah Jane was once that person.
We know the doctor doesn’t age and he doesn’t die. He changes in looks and personality, but he’s still the same man. That was already established in the new series. But the humans… they continue on as they are, they age, and they die while he changes — making new friends and new loves along the way. It doesn’t end for him, but it has to for them.
And yet, you see that even though The Doctor always falls in love with new people (in various ways), he doesn’t stop loving the ones he had to leave behind.