Vishnu prayed to Lord Shiva in all his forms, but Shiva didn’t pray to Vishnu. Why?
Your intent in posing this question is to get either a reply that Shiva is a greater God, most probably, or just to indirectly convey to Vishu-worshipers that Shiva is the supreme God or simply stir a debate on it!
Hinduism permits worship of multiple Gods who are endowed with different looks, powers and attributes; they, in reality, represent the One God, known as Brahman, Parabrahman, Paramatman or Satchidananda. Hinduism acknowledges that there are basic differences in every person in tastes, temperaments and capacity of intake in the matter of religion. In real life, a woman found distasteful to one man can be the soul stirring sweet-heart of another man. When such a difference is taste can exist, why not allow different tastes in worshiping the God?This is precisely the logic behind the idea of multiple God forms in Hinduism.
Thus, Hinduism permits you to choose a specific God form most appealing and lovable to you; it encourages you to believe wholeheartedly that that particular God form indeed is the one supreme God. A chaste woman considers her husband alone to be the most handsome and most wonderful person; likewise, at the lower steps of religion, a believer’s conviction that his personal God alone to be the most powerful and the “only true God” is also encouraged.
It is a common mentality amidst devotees of a specific God forms to think their God is the supreme God and all other Gods are subservient to Him. While it helps in focusing one’s devotion to his Ishta and get single pointed devotion, the negative side is that unless one develops in spirituality and grows up, he ends up in becoming a “selling agent” for one’s ishta!
Surprisingly, the same mentality seems to have existed in those sages who wrote specific Puranas like Vishnu Purana, Shiva Purana etc. The Shiva Purana, Linga Purana etc would claim Shiva to be the Ultimate God and Vishnu is a worshiper of Him. The Vishnu Purana will say vice Versa. The Devi Bhagavatam will say Shiva, Vishnu and Brahma were created by Devi!
If you are an ardent devotee of Lord Shiva and if you read Srimad Bhagavatam, particularly the episode about Vishnu taking Mohini Avatar and Shiva getting infatuated with Mohini, you would feel highly enraged and disturbed!
Puranas do serve a purpose. But if you take them too seriously, no specific spiritual benefit would accrue. Puranas can be used as a ladder at lower levels to step into higher level of spirituality so that the ladder can be discarded.