Money problems in our relationship.....!?

My boyfriend and i ahve been together for 3 years. We are both younger 19 and 21 i am younger. We live with eachother in a cute apartment He pays rent and i pay phone , TV and power and we both go in on grocerys. We have lived w/ eachother for 2and a half years, The problem is i dont mind buying things for him but when we go to a store and i get something for myself like clothes or makeup he wants me to "pay" him back? I think that this is rediculious! i would buy him things with out exspecting him to pay me back. When i go shopping i ask him if there is anything i can get him or i will call and tell him that (blank) is cool and i think he would like it, do you wnat me to buy it for you? I guess what i am getting at is this ok or should buying things be mutal? or his money is his money or mine is mine? Ide really perfer someone more mature to answer who has gone through this... A mans point of view would be great! We make good money so its not a money thing. Thanks for you input!