This cycle of meditation is believed to be because Brahma is a creator of nature, but before creation destruction is required, so he needs to understand the destruction, that's why he meditates on Lord Shiva, Vishnu is a preserver, to preserve he needs to understand the creation, that's why he meditates on Lord Brahma , Shiva is a the destructor, so to destroy he needs to understand preservation, therefore he meditates on Lord Vishnu. read more