Why do people jokeling insult each other?

I think that is so rude and disrespectful. My foster daddy's niece was insulting my foster dad as a joke saying he was "ugly" and has ugly as feet. He takes it pretty well. They were just teasing but I didn't think it was funny at all. She is 15 and has sort of an attitude. Her sister jokes too but she is nothing like her sister. She is 13. My foster dad has a great sense of humor. He teases her about her being too young to wear heels. She was like, "Well I don't wear heels too much because it FREAKING HURTS!!!!" And she walked away to sit at a table. He was like, "Ha nice comeback." She didn't respond she just takes to my foster sister. Why do people jokining insult each other? Were all suppose to be kind and loving. If she did that to me, I would have cried. My best friend and her boyfriend insult each other jokingly.