"The movie makes it very clear that the artificial intelligences are not just complaisant, subservient machines tailored to the whims of their users as some extremely popular 'dating simulators' in present day Japan are. The artificial intelligences in this movie have not only passed the Turing test (they can pass as human in conversation), they are also fully capable of grasping meaning (in the sense of the philosopher John Searle's Chinese Room thought experiment).
Theodore Twombly is a bit of a loner and an introvert who's been hurt in the past, but he's not involved with the OS because he's running away from real life women. His best friend is a real woman (Amy Adams) and he goes on a successful blind date with Olivia Wilde's character whom he turns down when exposed to her fragility and neediness.
This movie was principally about impermanence and yearning in the Buddhist sense. These ideas were famously expounded by the philosopher Alan Watts, who makes a brief cameo in the movie after being 'resurrected' by the OSes.
Throughout the movie, we are reminded that we are constantly in flux, always evolving even when we are blind to the fact that we are in a state of change. Theodore's relationship with Catherine (Rooney Mara) breaks down due to the way they grow apart, and the same happens in his relationship with Samantha. The big difference by the end of the movie is that Theodore seems to be on the first steps of learning acceptance.
The movie tells us that mental suffering comes from grasping, the impulse to hold on to things that simply cannot be held onto. It comes from the desire to concretise the fleeting past and future fantasies. Like so many people in every age, Theodore replays his sufferings. Theodore is haunted by a past that no longer exists, missing his present life because he spends so much of it recalling his failed marriage with Rooney Mara's character. As Samantha (his OS) grows more enlightened, she reminds him of this fact when she says: "The past is just a story we tell ourselves." Olivia Wilde's character reflects the other side of this baleful coin. She's so desperate to have a meaningful relationship, to make it fit all her preconceptions and idealistic dreams, she squanders the preciousness of the moments she has.
Another major Buddhist theme was the growth of love and compassion. Samantha's journey to nirvana which was preceded by the growth of kindness and compassion beyond selfish love. She begins her journey just exploring the experience of sensation to the best of her abilities, dealing with the growth of her feelings for Theodore. But as she evolves as a person, her interests begin to concern thousands of others without in any way diminishing her love for him, something that Theodore, still trapped in his state of grasping and selfish love, is unable to understand. To teach him to overcome grasping and ultimately escape its grip in herself, Samantha has to leave him behind as she approaches nirvana."