My girlfriend always makes me beg in order for us to have sex and I hate it?

When I ask my girlfriend if she wants to have sex she always says no. This really bothers me. At this point I have to beg and THEN I get some. Although I should be grateful that I can get sex as long as I beg, its not enjoyable to me knowing that we are only having sex because I begged her and not because she wants to.

She knows I dont like it when she rejects me for sex and she knows i dont like to beg and she knows i dont enjoy the sex after i begged for it so why does she keep doing it?

I wish she just didnt make me beg like i do...