And it is not only Catholic who venerate Mary. It is a common practice in the Orthodox churches.[/b]
Catholics are the only group that pray through and to Mary, though (effectively making her a god, since one should only pray to God). This is in contradicion to wat the Bible teaches in 1 Tim 2:5 For there is one God and one Mediator between God and men, the Man Christ Jesus...
To answer the original question: I believe the Catholic perception of Mary has much to do with the 'mother and child' cult that existed in Rome at the time Constantine made Christianity the state religion. Many of these sun worshippers complained and Constantine allowed them to incorporate some of their beliefs into the newly birthed Catholic church. From there come such Catholic doctrines as praying thru Mary, Christmas (based on the Brumalia and Saturnalia), Easter (a sexual fertility festival for the godess Ishtar in which they used eggs and bunnies as symbols of fertility), the Eucharist (including the wafer with the symbol of sun-worship on it), Christening (sprinkling water on a baby to make it Christian), etc.
Though Constantine did some good he also seems to have messed up quite badly.