My relationships never seem to work

Right basically, my relationships NEVER work out. No matter what I do, and I mean no matter what I do. I try hard, always meet my girlfriend when she asks, take her to dinner, go out to movies, have the occasional snuggle at home under the covers, go for walks with eachother ect. ect. but for some reason it never seems to work out. I mean I keep getting told i'm handsome and fit and whatever else and that I would make a perfect boyfriend. But that's obviously bullshit (pardon the french) because my relationships never last! is this normal or something I dunno? I mean i'm going into the Royal Air Force as a pilot and i'm a trained killer, does that have something to do with it?