system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] What does the recruiting process for the space marine chapters entail?

What qualities do they look for? Who does the recruiting? Do the various chapters look for different things?
gpt
It varies by the chapter but in general they're all looking for strong young men who already represent the very best their people have to offer. A lot of chapters make their home on worlds where life is not easy. Environmental hardship, frequent warfare, it doesn't matter as long as there's a natural selection going on to make the natives hard and strong.

Most chapters have a periodic recruitment run every decade or every few decades. Some chapters have the locals organise contests where their fittest men compete in a variety of events under the watchful eyes of space marines. Other chapters simply look for men who have been successful in battle or who have other achievements of physical excellence.

As long as the candidates meet the physical requirements, the marines care little about their personalities. Psycho conditioning takes care of that. 

For example the blood angels recruit on Baal, an irradiated wasteland of a planet. The Space Hulk game describes the various members of the terminator squad and they're quite diverse. One man used to be a psychotic gang member while another was a wasteland nomad. One of them is an artist while yet another is so dull witted that the rest of the squad jokes he's a servitor promoted by mistake.

In the end, the main quality candidates need is the physical peak condition to survive the surgeries, gene therapy, psychoconditioning and training that brings them from initiate to neophyte to fully fledged marine. Even the exemplary men selected by the marines usually don't survive.