10. Susan Leaving ("The Dalek Invasion of Earth", 1964)
The Doctors first companion, his granddaughter Susan, met a young man and fell in love with him on Earth in the aftermath of a Dalek invasion. The Doctor knew she couldnt, and shouldnt, follow him on his journeys forever, so he made a hard, sad choice and left her behind, locking her out of the TARDIS. It was the first time a companion had ever left, and though the Doctor left her on the shattered Earth for a good reason, that doesnt mean that it wasnt something deeply bittersweet.