The professional literature is replete with expressions of resistance to psychotherapy integration. Most of the literature highlights the pragmatic, methodological, and underlying philosophical differences that impede integration. Although these arguments possess validity, there may be other reasons to explain the pervasive resistance to integration among mental health professionals over the decades. One of these reasons is the human instinctual drive toward territoriality and demarcation among clinicians. Aspects of ego identification and a territorial imperative are hypothesized to be at the core of some resistance. If the human instinct of territoriality is indeed one cause, such resistance may be difficult to change and may forever limit the potential and acceptance of psychotherapy integration, or even the appreciation for what other modalities have to offer.