Why does my iguana like my sister more then me?

ok so i bought my iguana in september and i always try to pick him up my self but he always whips me with his tail but when my sister trys to pick him up he does not whip her at all!?!?! y is that i mean i do every thing for him he sees me every day and i juss dont get it help me please bcuz when he gets older i wanna play with him and everything with out getting whiped