1. The lack of a real "competitor" in a geopolitical sense for the West. I don't think it is a coincidence that the US arguably had the most competence and scientific development circa ±1940 - 1990, when it had a direct competitor to measure itself against. Incompetence against the Nazis or Soviets got you killed, whereas today, nothing really happens (immediately.) The ancient Greeks (and subsequently Nietzsche) recognized the usefulness of competition toward keeping yourself sharp and called it agon. Ironically the rise of China can be seen as a good thing from this perspective.
https://en.wikipedia.org/wiki/Agon
http://www.nietzschecircle.com/v3/2017/03/17/book-review-ago...
2. The loss of a civilization-wide teleological narrative. For the past ±1,500 years, that's been Christianity, in various forms. These days, there is no real "purpose" to the West, beyond some vague notions of individualism, choice and comfort. Institutions and standards become more and more difficult to maintain as a sense of apathy and the lack of a perceived future grows. Younger people especially seem unconvinced that there is anything waiting for them in 50 years.