As a longtime associate of Rocket though, the raccoon is able to understand what Groot is saying from the moment they're introduced. ... According to Guardians of the Galaxy director James Gunn, someone will only be able to understand Groot by spending time with him.

Why is rocket the only one who understands Groot?

Why does rocket understand Groot? Rocket can understand him while no one else can is because Rocket is an enhanced raccoon, thus giving him vastly increased sense of smell, to the point where he is able to distinguish between Groot's pheromones and use this to understand him.

Does Rocket speak Groot?

Groot's limited vocabulary was mentioned in Guardians of the Galaxy when Peter Quill questioned the figure's speech. Rocket explained that his friend's physiology rendered him unable to speak effectively. Despite this, Rocket was able to understand Groot's catchphrase as if it was a different language.