Why does Gatsby say he is telling Nick about his life?
In chapter four, Gatsby drives Nick to Manhattan for lunch as a friendly gesture, but his underlying motive is to attempt to shape the way that Nick thinks about him. Gatsby understands that there are many rumors perpetuated by the people who come to his parties and others in West Egg. He tells Nick, "I don't want you to get a wrong idea of me from all these stories you hear." Gatsby knows that Nick is Daisy Fay Buchanan's cousin, and it is important to Gatsby to find a close ally to help him in his quest to win back Daisy's love. He invents details about himself to win Nick's trust and sympathy--and because he wants Nick to believe that he is worthy of a woman of her caliber. Some of his claims are outlandish--like hunting big game in all the capitals of Europe--but some will prove to have elements of truth: his WWI exploits, his Midwest upbringing, and his time at Oxford. Gatsby is poignantly honest when he tells Nick "I didn't want you to think I was just some nobody."
Fitzgerald, F. Scott. The Great Gatsby. Charles Scribner's Sons, 1925.