After he returns from the war, Nick starts working in the bond business. He is looking for a modest place to leave in outside the city. Nick cannot afford the places at the Eastegg, but luckily he finds a cottage at the Westegg. By chance, Gatsby's house is next door. Most probably Nick's house is affordable given its size and simplicity, which is what Nick looks for, as compared from the other houses, particularly Gatsby's house.
