Because we are born defenseless and vulnerable, our primary and first survival strategy is to form relationships with our caregivers and the world around us. But sometimes, because of complex social structures, socio-economic or political forces, competing agendas, and our own personalities, we are prone to emotional "mis-attunement", meaning that our environments fail to provide a safe and reassuring experience.
Those experiences are inevitable.
And this means we adapt and learn strategies of survival to help us get through those tough moments. We might learn that the world is unsafe, so we do not trust it. We might get told that we are no good, so we believe we are no good. We might not feel validated or valued as a human, so we treat ourselves or others poorly.
Which means we might unintentionally sabotage ourselves or others; invite or seek excessive conflict; or get stuck in old familiar habits that always end the same way: painfully.
... every relationship would be a therapeutic one. But different relationships meet our needs differently, and sometimes we don't have access to the support group we'd like, or have another space to speak safely.
Therapy provides the opportunity to look at the emotional/mental parts of our lives, narratives, and experiences that we often look away from or would rather avoid, because of the bad feelings often associated with the coping strategies we learned -- all in the context of a professional and therapeutic relationship built to attune and provide safety for us to be ourselves.