score:15
I'm pretty sure the belief is not that earthworms become mothers, it's that every earthworm has most likely been your mother at some point in the past, as per the mata sutta (SN 15.14-19):
At Savatthi. There the Blessed One said: "From an inconstruable beginning comes transmigration. A beginning point is not evident, though beings hindered by ignorance and fettered by craving are transmigrating & wandering on. A being who has not been your mother at one time in the past is not easy to find... A being who has not been your father... your brother... your sister... your son... your daughter at one time in the past is not easy to find.
"Why is that? From an inconstruable beginning comes transmigration. A beginning point is not evident, though beings hindered by ignorance and fettered by craving are transmigrating & wandering on. Long have you thus experienced stress, experienced pain, experienced loss, swelling the cemeteries — enough to become disenchanted with all fabricated things, enough to become dispassionate, enough to be released."
So, the point is that the love and care we have for our mothers should really extend to all beings, since we are all intimately related - a corollary is that your boy/girlfriend has also probably been both your mother and your father at one point :)
Upvote:7
This question assumes frame of reference different from the one Buddhism operates in. The point of teachings is to produce a certain state of mind, that will affect your behavior in certain way, that will eventually lead to better results, up to and including Enlightenment for you and the rest of the sentient beings.
In this case, the image of mother is used as an epitome of unconditional love. By visualizing your mother and generating the feeling of love to her, you generate a certain emotion. Then, you learn to project this emotion on other sentient beings. In actual practice you start with the people who are naturally close to you, like your friends and relatives, and slowly expand it until all beings are included. The movie uses the worms scene to vividly illustrate to the audience the sheer depth that Buddhist compassion/love goes to.
Worms are not your mothers :) Everything is your mother. Everything is sacred. Everything is love!