Having for the first time?

my boyfriend and I ( im 17 and hes 16) are thinking about having and its obvious that hes ready and that he really wants to do it but ive told him that im not sure that i am..and he said that he'll wait for me..i know that we both love eachother and care abut eachother so i cant figure out why im not ready..so why is having for the first time for girls such a big deal..even wen theyre gonna do it with someone htey love?