It gives an explanation as to why Shukracharya was guru of Daityas despite being son of Brahma's manasputra Bhrigu and being as wise as Brihaspati. Yudhisthira also asked the same question to the beloved Bhishma Pitamaha as to,
“Why the renowned Shukra hated the Devas and favored the asuras. Who is this great individual and what is his history? Not many of us know about this.”
There were two primary causes of actions and footsteps which the great sage walked on.
Legends state that Lord Indra favored Brihaspati just like how Rishi Angirasa did and even had an alliance with him. Due to the alliance between Indra and Brihaspati, Shukracharya had ego problems with them and later, Brihaspati was even named as chief priest of Devas, which Shukracharya was not happy with. However, the son of Bhrigu was in many respects, a better choice than the son of Angiras for the position of the head priest of the Gods. Due to this Shukracharya started disliking the Devas and favoring the Asuras which was also a way to take the revenge of this partiality.
However, before Shukracharya’s alliance, it is believed that the Asuras used to be easily defeated by the Devas. Many Asuras used to be killed in the battle following the easy victory of the Devtas. Seeing this, Shukracharya decided to approach Lord Shiva for Mitra Sanjivani mantra using which Shiva could bring back the dead. This would mean a big problem for Indra and the Devas. Hence, Asuras fearing the Devas took shelter in his father’s ashram during that time. One such day, Shukracharya was far away from his father’s Ashram, doing tapasya for Shiva.
Finding an opportunity without Shukracharya’s presence, Lord Indra and few devas attacked Shukracharya’s ashram. However, Shukracharya’s mother Kavyamata gives protection to the Asuras with her yogic powers and Lord Indra gets defeated. Hence, Devas requests Lord Vishnu for help and Lord Vishnu comes to his rescue and thereby beheads Kavyamata with his Sudarshan Chakra.
Seeing this, Rishi Bhrigu, the father of Shukracharya and the husband of Kavyamata, curses Vishnu saying,“ Hey Vishnu! By killing a woman, you have breached Dharma and I, hereby, curse you to take seven births as a Human”. (However, later, Rishi Bhrigu breathes life into his wife Kavyamata.)
Due to the hatred, Shukracharya had developed towards Lord Vishnu and also Lord Indra for being one of the reasons for beheading his mother, he decided to become the Guru of Asuras. This a huge loss for the Devas as Shukracharya was highly knowledgeable since which, he defeats Brihaspati many times and revived many Asuras from death time and again with his Mitra- Sanjeevani’s knowledge.