Why does my boyfriend do that to me ?

ok so am 18 and my boyfriend 20 and is really protective of me when guys talk to me we went to this club on saturday and this guy came over talking to me this is while bu boyfriend we to the bar and my boyfriend was like telling him to get lost and that not the first time then i started shoutin at him asking why he dd that and he like coz i love you and am not haveing no guy takeing advantage of you and i see where he comes from but it quite scarey he doesn't mind me talking to my mates just guys he doesn't know he really weary of them he not controlling were he doesn't let me do out it just when were out together i love him loads but he gets me really upset when i see him go mad at other guys for just talking to me but he never has a go at me he says he trust's me but not them whyis he being like that my mates are like you should leave him but we been together 2 and half years and we practally live together and he really sweet with me buys me things please i need your help on what you think and how to deal with him thanks x ps am not flirting with theese guys they just come over and offer to buy me drinks but i do always say no and my boyfriend does belive me i love him loads please help