Eckhart Tolle explains dukkha, the Buddhist term for a persistent unease that comes from tying your identity to things that change: possessions, roles, relationships, achievements. When your sense of self depends on what's external, something always feels wrong.
Chapters
If you remain trapped in the conditioned energy field of the personality, then you are condemned to perpetual dissatisfaction, to put it mildly, or to use an even more generic term, unhappiness. That's a translation of a generic term that the Buddha used when he used the term dukkha, which can be translated, is usually translated as suffering, but in more generic terms, it means unhappiness, misery, unease. So you're condemned to that. Every situation turns into— eventually something turns up to mar the situation in some way, whether it's a person or place or an activity or something that you add to your life, that you obtain. There's always something. Whatever happens, there's always something That goes wrong, so to speak. But that's not a pessimistic statement. That is— that only applies if you are dependent for your sense of well-being, your sense of who you are, if you are dependent on all those external factors for your sense of who you truly are, your sense of identity, your sense of well-being.