My gf loves when is suck on her nipples, why?

It started with me doing it from time to time as foreplay, but now, she’ll specifically ask me to suck on them. She usually makes me look her in the eyes while I do it, and i don’t really understood what she likes about it. I like it, cuz boobs. But it really gets her going. But what’s like good about having your nipples sucked? Not that i think it’s weird, just wondering what makes it feel good. Is there anything else I should try, is there any technique to it?