People you counted on become unavailable, or someone you mistrusted shows a new side of themselves. Suddenly, you like the taste of raisins even though you hated them before. People are unreliable. The world is unreliable.
What then, can you trust in this ever-changing life? I think that the one thing that we all have, no matter what, is ourselves. In reality, the only opinion we can really change is our own. The only actions we can be sure to be carried out are the ones we perform.
This might seem depressing. But this theory also carries some comforting consequences. If we can count on ourselves, then this also means that we can choose to be there and support those we love when they need someone besides themselves. It also follows that, being the only ones we can rely on, we are not responsible for the behavior or choices of others.
If we believe that we can only rely on ourselves, then we are the only ones responsible for our own disappointment.