There is a story as to why Ganesha loves Durva grass. But before that, let us understand how Durva grass was born. As per the Vaamana Purana, during the Samudra Mathana, when Vaasuki was made the rope to churn the Milky Ocean, the devatas and demons pulled him on either side with great force. Due to this, Vaasuki was feeling pathetic. Some parts of his tail broke off and flowed near the shore of the Milky Ocean. These parts of the tail took root in the earth and transformed into the first shoots of Durva.
The Bhavishyottara Purana says that, during the Samudra Mathana, the cosmic hair from Kurma Bhagavan, who was holding the Mandara Parvata on His back, fell and Durva grass was formed. Therefore, both the accounts are true, but they might have happened in different Kalpas. Now, why does Ganapati love Durva? There is a story in the Skanda Purana and other Upapuranas too.
There was a demon by the name Analasura. As his name itself suggests, Anala means fire, and adura means demon. The fire-demon. The Upasana Khanda of Ganesha Purana (which is considered an Upapurana) says that when Yama got attracted to Tilottama, the celestial damsel, Yama’s veerya fell off, and from that itself Analasura was born. This Analasura was emitting fire from his eyes and mouth. So, he used to give a nuisance to the devatas. The devatas approached Lord Shiva. Lord Shiva instructed Ganesha to go and kill Analasura. Ganesha and Analasura fought fiercely, and when no weapon was harming Analasura, Ganesha swallowed that demon.
As Ganesha swallowed Analasura, the fire was causing a burning sensation for Ganesha. So, to remove that burning sensation, the devatas tried different remedies. Chandra came and sat on Ganesha’s head, due to which He was known as Bhaalachandra. Lord Vishnu put His lotus on Ganesha’s chest, but that also didn’t work as per His own will. Lord Shiva wrapped His cooling serpent around Ganesha’s stomach; that too didn’t work. The devatas applied rakta-gandha on Ganesha’s body and bathed Him with nectar; nothing worked. At the end, the sages brought the Durva leaves and put them on the head of Ganesha and chanted the Vedic Mantras. That eliminated the heat within Ganesha and cooled Him down.
Anyways, it came from Srihari only in His Kurma Svaroopa; it is the Bhagavad Prasaada, because of which Ganesha felt cooling. Plus, if we also take the Vaasuki narrative, since it came from the tail pieces of Vaasuki, it had the power to absorb heat. So now, Ganesha decreed that His pooja must be done by using Durva Patras. Like how, without Tulasi, Vishnu pooja is incomplete, similarly, without Durva Patras, Ganesha Pooja is incomplete. Therefore, Ganesha declared that whoever worships Him with Durva grass will be freed from all kinds of obstacles.
Ganesha is the worshipper of the Vishvambhara form of Lord Vishnu, as quoted from the Maandukya Upanishad by SriMadhvacharya. In this Vishvambhara form, Lord Vishnu also has an elephant head, with 19 human heads on either side. He has a broad chest and a slender belly, holding Vara and Abhaya Mudra. In the other two hands, He is holding Shankha and Chakra. So, because Ganesha worshipped this Vishvambhara form of Lord Vishnu through the stories in the Skanda Purana and Brahma Vaivartha Purana, He obtained the head of an elephant. So, this Durva also has the special presence of Lord Vishnu, or Lord Vishvambhara.
[Image of SriVishvambhara form of Lord Vishnu, which is always worshipped by Lord Ganesha]
Since He is the Upaasaka of Vishvambhara, He is also the presiding deity for the Aakaasha Tattva. May Lord Ganesha, the destroyer of all obstacles and the provider of intellect, provide us with good health, wealth, intellect, and devotion in Srihari, Vayu, and Gurus, and remove all our obstacles.
Hope this helps!
Jay SriRam!
SriGanapati Anatargata SriBhaaratiramana Mukhyapraana Antargata SriVishvambhara Abhinna SriKrishnarpanamastu!