I don't understand how enduring physical pain helps individuals of Eastern faiths reach enlightenment or serenity.
A couple of points need to be made here. I don't feel that enduring physical pain is something limited to "Eastern" religions. The concept of Lent for Christians is something rooted in self denial and a certain level of pain having to be endured. When Jesus spends 40 days and nights in the desert, he does so through an excruciating amount of physical and emotional pain. This is something that has to be replicated by Christians during the Lent period. The Crucifixion of Jesus is another example of where one is taught to revere the physical pain Jesus endured for the salvation of mankind. Here again, physical pain is used to further the connection between mortal and the divine. Another example of physical sacrifice in Western religions would be the concept of circumcision. In this instance, physical sacrifice is seen as integral to religious practice and faith. In these settings, I do not see physical sacrifice or pain as something limited to only the religions of "the East."
In trying to ascertain why individuals do what they do in the name of religion, I think that specific practices have to be identified and their religious meaning has to be explained in the context of religious faith and practice. If we cannot specify an exact example of where someone in the "Eastern" faiths endures physical pain, one can only speak in generics. Accordingly, I think that the physicality of worship helps to bring a devotee some level of solace in their relationship to the divine. Few, if any, reasonable people would suggest that they enjoy "physical pain." Yet, if there is a practice, tradition, or rite that a religion demands, devotees might take some solace in making this sacrifice for the religion that they love so very much. The physicality of this might be something that the devotee writes off as representative of their faith, or might be something that they believe serves as penance or allows them to have a closer relationship with God. In many respects, it is a sacrifice. It is one that is no different in any relationship of love. There are many sacrifices that we make for the ones we love in our lives that are painful. Sometimes, they result in physical labor or physical pain. In the end, we rationalize these because of the name of love and in much the same way, religious sacrifices of the physical nature might be seen in the same way.