I thought Tweety was a girl until some episode made it seem like Tweety was a boy. Maybe it was Tiny Toons that revealed it. If you look up Tweety's gender, Google shows a lot of sites saying "male." This result I took a screenshot of is interesting and teasing:
Groot can only say "I am Groot". When asked if the universe is worth it (a deep question I assume, philosophical, and sort of like asking someone what the meaning of life is, and that's not easy to answer) all he could say is "I am Groot", and that really isn't an answer at all now, is it? And it's a rather simple answer for such a grandiose question. However, there are much more to his words, and unlike his limited vocabulary his thoughts are much deeper and speak louder.
I'd imagine that Groot takes in this question, looks back at his experiences, the past, his feelings, what he's seen and knows, and responds with "I am Groot". But what is I am Groot? To other characters this has no particular meaning, but in Groot's mind and we the reader (who see his thoughts being visually expressed through his words) see much, much more. With those three simple words he expressed more than any words ever could. We see action, fun, friendship, respect, love, warmth, and other things being expressed, and to Groot it's what makes the universe worth it.
TL;DR: Groot can't say anything other than "I am Groot" so the writer/artist decided to give us a glimpse of what exactly he really meant by illustrating his thoughts onto his words.
@teerack: The series has illustrated inside of "I am Groot" in other issues as well. This one has the biggest meaning.