When his best friend dies in a drive by shooting, a naive vagrant must escape the harsh streets of his home world to join the galactic armed forces, endure their harsh training and battle nightmarish aliens to become a space knight.

The causal link between event (friend murdered in drive-by) and goal (become space-knight) is unclear. How is the latter response to the former?

Is it because, in the rules of your story's universe, the space-knights are some sort of police force, and with his newfound space-knight powers he's going to chase down and catch his friend's killers?

Why must he become a space knight?? Isn't that merely a milestone along a path to his ultimate objective goal?? Isn't his ultimate objective goal to avenge the death of his best friend?? To develop the skills he needs to defeat the bad guys?

Luke Skywalker accepts Obi-wan's call to become a Jedi? knight not for bragging rights or status.? Or to score with women.? Or to use a cool light saber.

His objective goal is to avenge the death of his step parents by defeating the evil Empire.

The logline for this story needs to be framed accordingly.

Why must he become a space knight?? Isn't that merely a milestone along a path to his ultimate objective goal?? Isn't his ultimate objective goal to avenge the death of his best friend?? To develop the skills he needs to defeat the bad guys?

Luke Skywalker accepts Obi-wan's call to become a Jedi? knight not for bragging rights or status.? Or to score with women.? Or to use a cool light saber.

His objective goal is to avenge the death of his step parents by defeating the evil Empire.

The logline for this story needs to be framed accordingly.