St. Paul told the early Church to pray to those who had died and were in heaven for help. Mary is surely one of the most saintly souls in Heaven. We pray to her to ask her for help. We ask her to petition her Son on our behalf.
Why?
“15 For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. And by him we cry, “Abba, Father.” 16 The Spirit himself testifies with our spirit that we are God’s children.”
If you can call God "Daddy", why do you have to ask Mary to petition Jesus on your behalf?
"St. Paul told the early Church to pray to those who had died and were in heaven for help."
Where?