The behavior suggest that: He is willing to do anything to achieve his goals.
Gareth realized that someone with a background like him is almost impossible to enter knighthood.
He took the job because he wanted to get Close to King Arthur and he eventually notices Gareth Value and potential.
Growing in height
growing in weight
or developing in puberty
It means that he has abided by certain mores for his whole life and by using the literal example of “pulling a thorn”, he’s saying, “there’s not one person who i haven’t helped, that’s what makes us brothers. When they need me i help them, and when i need them they help me.” It is interesting because he’s explaining that just because it’s not normal for you doesn’t mean it’s not my version of normal.