It is because you are her son that she would bite you intetionally. Just to show you how affectionate she has become of you! I guess everybody getting in contact with her has a chance to get bitten by her. ^_^"
In such a case you have three options:
#1 Wear protective gear.
#2 Clown costume. (Indeed creepy in many aspects!) >_<
#3 Bite back and show Setsuna your affection for her.
*Licks the bitemarks made by Setsuna.* ^_~