This question refers to The Great Gatsby, Chapter 7.
Answer: Nick suddenly remembering is his thirtieth birthday works as an unexpected warning to himself about the unhappiness that lays behind the world he has involved himself in since he moved to Long Island.
Step-by-step explanation:
After Gatsby begins an argument with Tom about Daisy, the infidelities and hostilities that have been long building up, finally explode, showing the real unhappiness the group shares. Nick fears that´s all his future holds.