The reason she chose Bayley for her partner was really the entire point of the film. Although they don't hit you over the head with the answer, and don't even have an "ah ha" moment where everything becomes clear, the answer is there, and it is a simple one. He was in her circle of friends, and was intellectually adequate to be a companion, but was socially challenged and not very experienced. What she came to realize was that he accepted her completely and without judgment. With this complete acceptance, she could trust him with her most private thoughts. In a pivotal scene, she lets him read her first novel, something nobody other than her publisher had been allowed to do, and then states rather matter-of-factly that they should have sex. This is the moment he went from one of her companions to her lifetime partner.

We find out through the course of the movie that she chose wisely, as he did, in fact, stick by her long after most would have had her institutionalized. Shortly before the end, when she has had nearly no lucid moments in a long while, her eyes suddenly came alive, and she said, "I love you." Clearly, this is to emphasize the point that the story is really about love, not a terrible disease, or the life of a famous novelist/philosopher.

I think the film would be an easier watch if you see it as a docudrama about Alzheimer's. Certainly the effects of the disease on her are disturbing, all the more so because of who she once was. The portrayal of this disease was so accurate that the Alzheimer's Association gave their entertainment award to Miramax for this film. In the introduction at the award ceremony, they talked about what a frank portrayal of the disease it was, and then said, "and it isn't even about Alzheimer's ... it is a love story." In accepting the award, Jim Broadbent shared that he had lost his own mother to Alzheimer's, and offered a personal anecdote. Shortly before her death, when she hadn't been coherent for months, his sister said, "Mom, we just love you too much." The lights came on for the last time, and she answered, "You can't love someone too much." Seeing it as a love story, I was devastated by what her husband was going through. She, clearly, was the light of his life. His love was strong enough to stick by her to the end, but his source of strength, Iris, was mostly not there. Only Iris knows if she was suffering in the later stages of the disease, but Bayley had all, or at least most, of his faculties.

The film is great, but it won't please those hoping for a biopic, and is emotionally a very hard film to watch.