In Avengers Hulk tried to raise Thor's axe and failed miserably. And in Infinity-War Groot raises it and holds it for a few seconds! Is new Thor's axe made of some other light material, or is Groot stronger than Hulk?
The hammer Mjolnir, Thor's previous weapon, had an enchantment placed upon it by Odin so that it could only be wielded by those who were "worthy" of doing so. Hulk and the other Avengers (with the exception of Vision) couldn't pick it up not because they lacked the physical strength, but because for reasons we're not privy to, Mjolnir deemed them unworthy.
The axe Stormbreaker never had that enchantment placed upon it, something confirmed by the Russo brothers themselves. Therefore, all that's required to lift it is physical strength.
@F1Krazy Perfect, Stormbreaker didn't have any spell(s) bound to it, it was a weapon made out of Uru. However, notice how Thor tells Starlord that "your bodies will be crushed as your mind collapses into madness" while referring to his new weapon (turns out to be Stormbreaker).
So now I ask "if not the guardians, why Groot specifically?" Well, Stormbreaker wasn't formed completely first of all. Secondly we can CLEARLY see teen Groot in pain and feeling the burn when he uses his arm and chops it off to complete the weapon-building.
The axe became the actual Stormbreaker AFTER Thor channeled his lightning through it.
Also, picking up a weapon for a few seconds is NOT EQUAL to wielding it for long durations, and DEFINITELY NOT EQUAL TO using it in any battles like Thor did!!
Providing a ref link as well, here you go !!---> https://screenrant.com/avengers-infinity-war-groot-thor-axe-stormbreaker-explained/
Groot is worthy as per Odin's user credentials i.e whom ever be worthy shall weild the power of Thor Vision is also worth in layman's terms no hidden agenda just the will to do good.