One might say, "No film with a plague of frogs can be all bad." But in fact P. T. Anderson's Magnolia is is a good film. It is marked by its day, like any film, and its obsessive preoccupation with self seems dated now, given what has transpired since 1999.
Magnolia on the one hand illustrates the effects of serendipity, of seemingly random and unrelated acts that become related in an arbitrary way and have an impact on our lives. The effect is as if there is some divine hand out there, controlling events, shaping destinies. But I don't think divine intervention is what the film means to imply. On the other hand, one could argue that there is a strong religious or spiritual sentiment in the film.
Magnolia gives us an array of examples of human misery and suffering: incest, addiction, parent –child estrangement, disease, abuse, loneliness, suicide, adultery—especially loneliness. The characters as the film progresses appear increasingly desperate. In terms of the film's symbolism, Earl Partridge, an old and dying father played by Jason Robards, is a center. He suffers throughout the film, always on the verge of death. Everyone in the film, either actually or by implication, is somehow involved with him. He is the central image of human suffering, and of the possible redemptive or transformative effects suffering can bring.
At a key moment late in the film, the character Jimmy Kurring, a religious police officer, is driving down a city street and a large frog falls on the windshield of his car. Soon frogs seem to be falling everywhere, throughout the city—an unnatural, inexplicable event. A veritable plague of frogs. Once again, serendipity. One could speculate over the meteorological reasons for the event—and P. T. Anderson gives us a world in which there are always explanations, even if unlikely ones. But the plague of frogs in Magnolia functions as if it is God's judgment on the living and the dying, on all that suffer.
To confirm this effect, soon after the frogs stop falling all the main characters, individually, join in singing "Wise Up," a song written and sung by Amy Mann. It is as if to say, "We all suffer." The first time I saw this film, I was deeply moved by this scene. Now, it is still moving, though it also seems stale and slightly contrived.