One of the coolest things about the first Guardians of the Galaxy was how director James Gunn had Vin Diesel voicing Groot. The character has two lines so to bring in an actor with a voice and presence like Diesel gave everything gravitas. But, things may be changing for Vol. 2.
Source: io9 – James Gunn Hints at Baby Groot’s Voice in Guardians of the Galaxy Vol. 2