Guardians of the Galaxy attempted the Herculean task of simultaneously introducing a big group of ragtag characters and making them lovable, both as a group and as individuals. James Gunn and the rest of the crew succeeded, instantly making some of the weirder members of Marvel's canon fan favorites on a massive stage. Groot is a massive humanoid tree, but the strangest thing about his character is his dialogue.
Marvel Comics authors have had a lot of time to come up with strange aliens. Most of them appear for an issue or two, only to be forgotten. That was Groot's initial fate, but the other commonality among the cast is that most characters will eventually be reused. Groot came back with a vengeance, and now he's one of the most popular characters on the big screen.
Where did Groot Come From?
Groot is a Flora colossus, and typically the last of his kind. He hails from Planet X, which is the capital of his people's empire. On Planet X, noble-blooded "Arbor Masters" are in charge. They teach the children and make the rules. A species of squirrel-like rodents called Maintenance Mammals help the tree beings to get by. Groot was more compassionate than most of his kin, eventually leading to his exile. He happened upon a fellow sapling savagely beating a Maintenance Mammal and killed him to save the defenseless animal. This got him exiled from his home, but the punishment was a blessing in disguise. Groot set out to wander the galaxies while his home planet was obliterated by Ronan the Accuser. The growing tree spent years on his own before finding and forging a partnership with Rocket Raccoon.
Why can Groot only say "I am Groot?"
The films and the comics offer two slightly different explanations for Groot's dialogue. In the original Guardians of the Galaxy, Rocket explains it as if it's an issue of intelligence. Rocket states that "he don't know talkin' good like me and you." He can only say "I am Groot" and others can understand his nuanced intonation after spending some time around him. Rocket has been around him long enough to fully communicate, but it takes Star-Lord and the other Guardians a while to pick up the ear for it. Thor immediately understands Groot when they first meet in Infinity War, offering an Asgardian college elective as an explanation. This slightly contradicts the original explanation, but James Gunn has suggested that the God of Thunder was making a joke. Instead, Gunn offers his deific senses as an explanation for how he figured out Groot's language. Either way, it appears to be an acquired skill.
In the comics, Groot's language is explained through more biological means. Flora colossi don't really have a respiratory system, but they do have some matching parts with human beings. Most notably, they have a larynx that seems to possess some form of vocal cords. According to the Inhuman Maximus the Mad, the adult Flora colossi develops a stiff larynx, which is only capable of emitting certain sounds. This doesn't make any sense, since the larynx only produces sound, and the words formed by those sounds are largely controlled by the mouth, but that's the justification they go with.
Maximus goes on to assert that Groot's thoughts are actually very nuanced and that only a well-trained ear could discover the subtle inflections of his voice. Psychic Jean Grey later discovered that Groot is actually extremely intelligent, and well-versed in such complex subjects as "quasi-dimensional super-positional engineering." Maximus, though unquestionably and openly insane, seems to be right. Groot is saying a lot of things, but his unique physiology causes the uninitiated to hear "I am Groot" every time.
Is The Groot in Vol. 3 a Different Groot?
The Groot that will appear in the third chapter of the Guardian's quest will not be the same being as the one introduced in the first film. He is the same one introduced at the beginning of the second film. As those who saw the original Guardians of the Galaxy know, Groot willingly sacrificed himself for his new friends, resulting in his tragic death. His best friend Rocket picked up one of the splinters he left behind and planted it, allowing it to grow into a new Groot. This Flora colossi, colloquially known as Baby Groot, is the one that has starred in every film since then. He was depicted as an infant in Guardians 2, a teenager in Infinity War, and he'll be a full-grown adult in the upcoming end of the trilogy.
Groot is a unique character with a lot of strange details. He may seem like the comic relief character of the group, the guy with nothing to say, but he's actually one of the most engaging conversationalists on the team. Just because we can't understand every "I" and "am" and "Groot," doesn't mean he doesn't have something worth saying.