The answers are quite different:
For couples, it’s usually because (at least) one of the partners is resistant to owning their part, or secretly wanting the marriage to end.
For individuals, it’s usually due to one of:
- quitting too early
- selecting a poor therapist or an ineffective healing modality
- homeostasis preservation (aka unconscious resistance or sabotage due to fear of [unpredictable] change)
What are the benefits of seeking therapy?