Wikipedia says that one of these may result from any one (or several) of these:
- The sense of being alone and isolated in the world;
- A new-found grasp or appreciation of one's mortality;
- Believing that one's life has no purpose or external meaning;
- Awareness of one's freedom and the consequences of accepting or rejecting that freedom;
- An extremely pleasurable or hurtful experience that leaves one seeking meaning;
Not really sure which one I better fit into. But it's on.