Because she's Mum

Luke Smith was not an ordinary boy. He didn't grow up playing with other kids; trading Pokémon cards, playing computer games, and messing up his room. He didn't groan about school, pull girl's braids, or draw on his walls with crayon.

Luke Smith was born at 13 years of age, and he was born running. He'd been created by an alien race called the Bane, in order to test the taste buds of every human that walked into the Bubble-Shock! factory. But that's a story for another time.

He may have been very different from other children, but one thing that he had in common, was that he loved his mother. Of course she wasn't his biological mother, but she was still his mother. She wasn't anything special to the ordinary people walking the Earth. But to those who knew what she really did, she was absolutely irreplaceable. And she was the woman who had adopted the lost boy who didn't know who he was.

Sarah Jane Smith was a woman of many personalities. Her curiosity, determination, and that special quality that only she had, was what Luke absolutely adored about her. She could make anyone feel special, and the best thing was: she never even realized it.

With an alien super computer in her attic, a genius son, and the memories of traveling in time and space, she was definitely not the typical human being. Throughout her various experiences with aliens that she shared with Luke, as well as his friends Clyde, Rani, and Maria, Sarah Jane showed compassion for not just humans, but for any lost soul from anywhere in the universe. This was just one of the many things that made her who she was.

Danger was of course, what you'd call an occupational hazard. Getting transported to an alien space ship, running from Slitheen, and getting kidnapped by nuns, was something that didn't really faze Luke or his mother. Though neither of them ever mentioned it to the other, they were always nervous when a new threat revealed itself. Not because they were scared for themselves, but because they each knew that the other would go into whatever danger arose, ready to save the world.

The Smiths were constantly saving each other, and Luke was ready to face any danger to save his mother. When she had gone missing after the arrival of an alien in disguise (who went by the name of Ruby) Luke had panicked. He'd lashed out at Rani, terrified that she and Clyde had done something to chase Sarah Jane away. When he'd discovered that Ruby was holding his mother captive in the basement of an old building, he'd seen red. His mother would be scared, lost and alone. He'd ended up threatening Ruby, and helping to save his mother.

Luke Smith would do anything in the world for Sarah Jane. And if anyone was ever blind enough to ask why he was always such a devoted son, he'd given a simple reply.

"Because she's mum."