The first time Groot noticed dancing, it demanded his complete and whole attention more than anything else he had ever come across. Mind you, the key word was notice. Any other time held something far more interesting or his short, one-of-a-kind companion was loudly requesting he do (or not do) a particular action, to which he would usually follow. There were times he would even follow requests although they weren't specifically directed to him at the time.
What caused him to be so entranced in that particular moment took up two of his senses and involved more than the usual being shimmying their body around. Peter Quill was much more to Groot than the usual being. And he was more than just shimmying. Limbs flew around in perfect sync with the beat, flowing with the melody of a song blasted loud enough on the ship for the sparse leaves on Groot's body to tremble slightly. He cocked his head in observance.
The moment was broken when Gamora yelled for the Terran native. The music, however, continued on as he bounded away to her, head bobbing. Groot then walked cautiously to the empty spot, processing what he just witnessed. Listening for a few seconds to the new song playing, he then tried to imitate the man's moves, but alas his limbs were not limber enough and his body simply refused to follow the same motions. Or speed.
Groot rumbled in the back of his throat in displeasure. Branches were not made for such minute movements. All he could accomplish was waving his arms wildly about. Having his leg join them would result in unbalancing and thus keeling over, something he did not enjoy. Legs stayed firmly established on the metal floor.
After a few more attempts at arm waving, he gave up and lumbered in search for his mouthy companion.
That was before the encounter with Ronan. Currently an essence of him was firmly planted in a pot, slowly growing in consciousness. It faintly remembered a time before being rooted in soil, striding along with the same mouthy creature (who was much smaller in past recollections). It also remembered the music which was currently reverberating through the green wood of its being.
When the resurrection of Groot had gained enough awareness, as well as familiarization with Peter Quill's dance moves, he decided to try the concept of dancing again. With the memory of the last attempt in the back of his consciousness, he began bobbing his top to the beat, shortly followed by his body swaying along. Arms quickly joined the movement, not in imitation of the moves he saw before but in his own pattern. His new size helped in keeping up to the rhythm and he began to enjoy his simple revelry in the music.
Of course, it would not do for anyone else to witness his newfound rigorous entertainment just yet. When he sensed Drax turning to stare intensely from behind he immediately froze, then resumed his new favourite past time once the pressure left. He would keep this a bit longer to himself, then reveal it at the opportune time.
Groot couldn't wait to see the reaction on his companions' faces.
A/N: I imagine Groot to be a closet dancer, considering the credit scene.