At the end of the hotel scene, Nick says something that I have always found so strange. He says " I just remembered that today's my birthday. "I was thirty. Before me stretched the portentous menacing road of a new decade." Why would the decade ahead of him be menacing

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.

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.

Nick said it because of his previous experiences which were not so easy to handle and that have transformed his thoughts so much that he considered that his future would be same as his past. He even didnt remember his birthday, indicating that his mind is well occupied by the menacing experiences and thoughts. Since he has turned thirty so now people think its time to settle but for Nick its still a road of struggle ahead of him