I just rewatched the first Guardians of the Galaxy movie and this part wasn't well explained. Why would Groot die just because he wrapped up his friends? He was already shown to be able to expand/stretch his body with no repercussions, but as soon as he starts to expand to encircle his friends in a protective tree wrap Rocket says "No Groot, you'll die!"
Did I miss something, why would changing his shape into a protective sphere kill him?