Living in Oakland at the beginning of the 20th century, Martin Eden struggles to rise above his destitute, proletarian circumstances through an intense and passionate pursuit of self-education, hoping to achieve a place among the literary elite. His principal motivation is his love for Ruth Morse. Because Eden is a rough, uneducated sailor from a working-class background and the Morses are a bourgeois family, a union between them would be impossible unless and until he reached their level of wealth and refinement.
Over a period of two years, Eden promises Ruth that success will come, but just before it does, Ruth loses her patience and rejects him in a letter, saying, "if only you had settled downÂ ... and attempted to make something of yourself". By the time Eden attains the favour of the publishers and the bourgeoisie who had shunned him, he has already developed a grudge against them and become jaded by toil and unrequited love. Instead of enjoying his success, he retreats into a quiet indifference, interrupted only to rail mentally against the genteelness of bourgeois society or to donate his new wealth to working-class friends and family. He felt that people did not value him for himself or for his work but only for his fame.
The novel ends with Eden's committing suicide by drowning, which contributed to what researcher Clarice Stasz calls the "biographical myth" that Jack London's own death was a suicide.
London's oldest daughter Joan commented that in spite of its tragic ending, the book is often regarded as "a 'success' storyÂ ... which inspired not only a whole generation of young writers but other different fields who, without aid or encouragement, attained their objectives through great struggle".

Question: Why does Ruth reject Eden?
Answer: Because Eden does not become wealthy and successful soon enough.  
Alternative answer: Eden did not settle down.