system
Given the following post, respond as if you're commenting on Reddit:
human
[Doctor Who] Why are the vast majority of the Doctor's companions young beautiful women?
gpt
At different points in his life he has different reasons. But all of them were grounded in loss.

Early on, he is trying to replace his granddaughter. For his first three versions, he often sees his companions as children to protect and share the wonders with.

After his third regeneration something changes. His new body is younger and more energetic - no longer trying to be the wise grandfather figure. This is because of his current companion: Sarah Jane Smith. Sarah changed the doctor by being much more of an adult friend - someone to confide in and who would actually take responsibility for herself in dangerous situations.

After this shift, he was more looking to replace Sarah. He had grown fond of her in more ways than he would like to admit, and tried to fill this new void in his life. This lasted until the Time War.

After the war, the Doctor was a broken man. Rose "fixed" that, and let him rediscover who he was. In her he found a person he could protect like the granddaughter he had almost forgotten about, but also a strong selfless person who reminded him of Sarah Jane. But more than filling the void in his heart, she added something new: compassion and a kind soul. Rose cared about the Doctor in ways he had not experienced since he lost his family, and he slowly started to return the feelings she were showing him.

Losing Rose devastated the Doctor. Now he had a new hole in his heart to fill, and he fell back into old habits. Donna Noble might have stopped him from going totally off kilter, but deep down he was still looking for the next "Susan", "Sarah" or "Rose".