There are a number of additional details worth mention. First, Vol. 2 happens shortly after the events of the original film. Since Guardians takes place far away from Earth and doesn’t connect directly to the larger MCU, Gunn was able to adjust the timeline to fit his story needs. Secondly, Gunn was actually concerned about pitching Baby Groot (and other strange characters) in the film.
“When I approached Marvel Studios with my idea for Vol. 2 I was afraid of the involvement of Ego the Living Planet – a rather, uh, broad character in the 616 universe of Marvel Comics. I was afraid of the inclusion of Mantis and Ayesha, and more prominent roles for Nebula and Yondu. But I was even more afraid of approaching them with the concept that Baby Groot, and not Groot, would be a member of the Guardians of the Galaxy. Yes, perhaps it seems like a hole-in-one idea now, but Groot was the most popular character in the first film, and sometimes when something works studios are more than a little reluctant to change.”
Fortunately, Marvel, like Gunn valued rich character development over the “safe” road. One of the reasons Guardians of the Galaxy connected with audiences was its lovable characters; watching them slowly bond and grow as a team added deep emotional layers to what could have easily been a shallow spectacle. Baby Groot was a necessary inclusion, in part because Gunn wanted to jump right back into the story and examine the Guardians evolution as a family, along with all the quirkiness, tension, and messiness that comes with it.