Why should we wear yajnOpaveeta?
YajnOpaveeta is the sacred thread that represents deeksha to perform vEdic rituals. Man belonging to the first three varNas (brAhmaNa, kShatriya and vaishya) has vEdAdhikAra. He can perform sandhyAvandana, gAyatri japa, agni kArya and the like and can also perform vEdAdhyayana. The pre-requisite to all this is the formal initiation called upanayana where the boy’s father gives the yajnOpaveeta and brahmOpadEsha to his son initiating him into vEdic rites.
Nobody is entitled to perform any samskAra without having gone through the yajnOpaveeta samskAra according to ManusmRuti. One must undergo yajnOpaveeta samskAra before the vivAha (wedding) samskAra. Similarly, shrAddha karma is not valid unless the kartRu has first undergone the yajnOpaveeta samskAra. Even when a person does not wear a yajnOpaveeta regularly, priests insist upon it during such rituals.
The yajnOpaveeta is said to have been created by Sri chaturmukha brahma himself for this very purpose:
brahmAgrE kalpayAmAsa vEdArhAnAmanuttamam
yEnAmsE dhRutamAtrENa brahmasUtrENa tE dvijAH
bhavaMti brAhmaNa nAma vEdArhAshcha trayastviH ||
The thread that gives the eligibility to perform yajna kAryas is the yajnOpaveeta
yaj~jAkhyaH paramAtmAya uchyate chaiva hOtRubhiH
upaveetaM tatO AsEdyam tasmAdyaj~jOpaveetikam ||