It's not exactly that complicated of a theme. Mary is supposed to be the mother of God, and mothers breast-feed, so anything to do with breast milk will instantly have "motherhood" written all over it. As for why she had to be cartoonishly squirting the milk at Bernard from 5 feet away, well ... for one, it allowed both parties to maintain a respectful distance from one another, because these are two adult virgins we're talking about. There is church protocol to be followed! Good taste and morality have to be considered, and ...
"The Vision of St. Bernard" remained a popular theme throughout Europe until the Council of Trent brought the fun and games to an end after someone realized that artists were basically painting one guy's wet-but-not-in-that-sense dream.
Just like that time your mom opened your computer folder marked "Biology Homework."