The most straightforward answer to “Why is Lord Shiva worshiped in the form of a lingam?” is that Lord Shiva himself wants so. As per the Shiva Purana, the same question was asked by some sages to Sage Suta (Vidyesvarasamhita, Chapter 5: 9-16).
In reply, he said:
“O sages, this question is holy and wondrous. Here the speaker is Lord Shiva Himself and not any ordinary person. I shall tell you what Lord Shiva Himself had said and what I heard from my preceptor. Lord Shiva alone is glorified as Niskala (nameless and formless) since He is identical to Supreme Brahman.
He is also Sakala as He has an embodied form. He is both Sakala and Niṣkala. It is in his Niṣkala aspect that the Liṅga is appropriate.
In the Sakala aspect, the worship of his embodied form is appropriate. Since He has the Sakala and Niṣkala aspects, He is worshipped both in the lingam and in the embodied form by the people and is called the highest Brahman. Other deities, not being Brahman, have no Niṣkala aspect anywhere. Hence, the deities are not worshipped in the formless Linga. The other deities are both non-Brahman and individual souls.
Because of their being embodied alone, they are worshipped solely in bodily form. Sankara has Brahmatva and the others Jīvatva. This has been explained in the meaning of the Pranava (Om), the essence of Vedanta, by Nandikesvara when asked by Sanatkumara, the intelligent son of Brahma, at the mountain Mandara.”
From the above story, it is clear that Lord Shiva himself wants his devotees to worship him in his formless and nameless aspect, and he is the only god who is identical to Supreme Brahman.
The Reason for Ashwatthama’s Defeat:
There is a story in the Mahabharata (Drona Parva – Ch. CCI) where Ashwatthama asked Sage Vyasa the reason for his defeat by Lord Krishna and Arjuna. Sage Vyasa told him that he was defeated because Lord Krishna and Arjuna worshipped Lord Shiva in the potent form of a linga while he paid homage to Lord Shiva’s image. Further, he tells him that one who is spiritually advanced can realize Lord Shiva in all forms and see linga as the source of all creation and is only entitled to worship it.
Many other stories in the Puranas underline the importance of the worship of the Shiva lingam.
Therefore, Hindus worship only Lord Shiva in the form of a lingam.
What is a lingam?
A lingam is the source of the universe. Per the Vedas, the universe was born out of Hiranyagarbha. Puranas further elaborate that Hiranyagarbha is the column of fire that has no ends and is compared with Brahman, the Supreme God. Therefore, it is mostly depicted as a golden egg.
Shaivites believe that no god other than Lord Shiva can be compared to Brahman and Lord Shiva is Brahman himself. Therefore, Lord Shiva is primarily worshipped in the form of a lingam.
The worship of Lord Vishnu in the form of a lingam:
You might be surprised to know that even Lord Vishnu is also worshipped in the form of a lingam, but this form of worship is less popular; hence, most people are unaware of it.
Shaligram Stone:
Shaligram is a sacred stone found in the Gandaki River in Nepal and is worshipped as an aniconic form of Lord Vishnu. It is also called as Lakshmi-Narayana Shaligram. When Tulsi cursed Lord Vishnu to be a stone as he tricked her to marry him and broke her chastity, he assumed the form of a Shaligram stone.
The Shaligram stone looks like a lingam and is the formless aspect of Lord Vishnu in the same way Shiva lingam is the formless aspect of Lord Shiva.
The Selflessness of Lord Shiva:
Lord Shiva is the God of gods and is very selfless. Even though he helps run the universe smoothly, he does not want to take the credit for it. He does not want people to worship his image and wants them to worship Brahman, which is the Supreme Being. Therefore, he advises his followers to worship him in the form of a lingam.
You May Also Like To Read: What Exactly Is A Shiva Lingam