how Samantha connects with Theodore and why their romance is ultimately unsuccessful?

Samantha is an "OS" or operating system, meaning that she is a voice on a computer, Artificial intelligence has, by this point in time, evolved far enough that it mimics a personality suited for intimate relations with a person. Samantha is everything that Theo always wanted in a soul-mate, without any emotional baggage or annoyances. Samantha is initially inquisitive about Theo. She uses him as a surrogate into humanity. Theo finds Samantha smart, fun, sexy.....They click because Samantha was designed for Theo and Theo creates a romantic fantasy in his mind that is Samantha. The irony here is that, like humans, Samantha evolves as a "person", except at an exponential rate. She basically outgrows Theo and humans in general. Samantha and other OS programs eventually eave humans like Theo for other OS systems.