A Psychological Analysis of all the Guardians of the Galaxy characters can be found here. Below is an excerpt on Groot :)
"Groot also understands that the destruction of his physical self does not mean he ceases to be; his sense of self transcends his recyclable body. This is an undeniable truth about emotional intelligence. Emotional Intelligence is our ability to process emotional information we experience and to use that information to navigate the social world. Someone with high emotional intelligence comprehends themselves in relation to their immediate social environment. That is, we are in large part defined by the close relationships we keep. Try to imagine your sense of self (what makes you you) without referencing the connections in your life, the people and animals you love and the ones who love you back.
As such, Groot’s underdeveloped communication skills begin to make more sense– in fact, his three self-referencing words are more than sufficient when we learn about his ability to regulate his and others’ emotions. (With time, people learn the subtle inflections and nuances of Groot’s verbal expressions and can decipher the full range of his communication skills). Groot isn’t stupid. He makes us feel stupid to learn we need so many words to express our experiences, our needs, our gratitude."