Undeniably, he's everyone's favorite tree and since Guardians of the Galaxy Vol. 1 up to his latest appearances, he only seems to say the same thing over and over again, even having it as the title of his own mini-series, "I Am Groot." Why are his words limited only to those three?
Why Groot Only Says "I Am Groot" Explained
If you'd ask any Marvel fan what Groot says, it will always be "I Am Groot," because basically, that the only thing he could say, maybe except in Guardians of the Galaxy Vol. 1, where he saved the others and said, "We are Groot" but that's about it and yet, other Guardians understand what he says.
Well, the reason why Groot can only say "I Am Groot" is because he belongs to a race called Flora Colossi which makes Groot a Flora Colossus specie from Planet X. It is the capital of the branch worlds and they are tree-like beings who have stiff larynxes making their words difficult to understand.
This garbles the speech of the Flora Colossus species so they sound like they are repeating phrases like Groot does when he says "I Am Groot" but they mean something, apparently, since the other Guardians understand what Groot is saying.
To date, only Groot is the Flora Colossus character in the MCU and it is still unknown whether his entire race is extinct or not.
The reason why the Guardians could understand Groot is not through the translator implants they have but with how they interacted and bonded with him over the years as per James Gunn. This is proven on how Star-Lord adjusted before he could fully understand Groot.
Also, according to director James Gunn on Twitter, the crew and the cast have an understanding on what Groot means with each "I Am Groot" because he has scripts with the translations so Vin Diesel, the voice behind Groot, knows how to express each line.
The now-iconic line became the title of Baby Groot's own mini-series, I Am Groot, which, by the way, has been renewed for Season 2, so expect a lot of "I Am Groot" lines coming soon with the second run of the mini-series.