We don't understand each other.
Remember your Old Testament and the Tower of Babel? Everyone woke up one morning and god had given them all different languages so they couldn't communicate with each other and become greater than him. The characters in the movie are all trying so hard, but they don't get through to each other. There is one senseless near-tragedy after another, each brought on in its own way by a communication barrier, whether literal language or mental block.
The film is a good bit longer than it needed to be for my taste and it requires strict attention not to miss the subtitles unless you're fluent in Arabic, Japanese, Spanish and English and can hear sign language. Other than that, I recommend it as a thought-provoking movie.