Why do i feel this way for her?

im going out with this girl i been like for about 2 years even before i talked to her,when i seen her the first time i just liked her, when we first started to talk she would tell me about her past boyfriends so i could get to know her more and even now when we together she would talk about them or other guys but not alot,now i never been a jealous person but for some reason even when we werent together, whenever she would tell me about her and some guy i would get jealous,this never happened to me with my other ex girlfriends so i really dont know why its happenin with her,is it because i like her alot cus i really do. so why do i feel that way with her?