Am I the only one?

My longtime boyfriend and I have a great sex life, and we do it pretty often. During sex sometimes I get dry down there. And I'm in the mood and I'm enjoying what we're doing but my vagina will just get dry and it's hard for us to keep going. And it makes it hurt when he tries to go inside me because I'm not wet enough. Is that normal? Is there anything I could do to fix that or help it??