Why lord siva is associated with tantra ?

Siva and Tantra

Lord Siva is Pasupathinath, the Lord of animals. He is also the Lord of all animal passions that lay hidden in us. The belief is that if you pray to Lord Siva with complete devotion he will rid you of all your animal passions and change your consciousness into divine.

Tantra also aims to achieve the same end, not through the control of animal passions but through their controlled expression. In Tantricism the Siddha identifies himself with Lord Siva and indulges in various acts of self purification in order to merge with Siva, so that he becomes Siva in reality. He accepts Siva as the means and Shakti as the end.

Long before the emergence of Tantricism as a major cult in India, Siva was already identified as a fertility Go. His worship in the form of Sivaling was popular in various parts of India and also beyond. The Indus valley people must have followed some vague form of rituals involving the Mother Goddess and her male counterpart, probably a prototype of Siva, whose seals were found in the ruins of Mohenjodaro and Harappa.

The Sivaling symbolized all that Tantricism stood for. Tantricism merely provided the philosophical justification for the worship of Siva as a God of love and liberator of mankind through the worship of Shakti in union with Siva. Tantricism suggested a new way to approach the subject, but was not in conflict with the fundamentals of Saivism.

Quite harmoniously, it blended the agnostic and atheistic philosophies of the sixth century B.C. with the theistic school of Saivism, accepting Siva as the Purusha, the eternal and indivisible principle which the Sankhya Vadins and the Charvakas vehemently denied to acknowledge.