Why You Can't Win
Now, we're not saying that he's jealous because you're so incredibly awesome and dude, he's so incredibly lame. We're saying that no matter what age you are, everyone feels threatened by younger members of the same sex. This is why we have these creepy stereotypes of "daddy's little girl" and "mama's boy" -- parents tend to favor their opposite-sexed children. It's partly because we condition boys to be our competitors, and partly because we're all petty animals at heart. You showing up on your girlfriend's dad's doorstep is like slapping the old man in the face with his own flaccid manhood.
Keep in mind, every middle-aged guy has some regret. You will too once you reach his age and realize that you will in fact never have your own rap label. So when you show up to pick up this guy's daughter, it's not just that he sees the douche who's going to be boning his little girl later (which he does, but more on that in a minute). He also sees himself at your age, and all the wasted years in between.
Then again, maybe your girlfriend's dad is a billionaire skydiving globetrotter who's accomplished everything he ever dreamed of doing. Maybe you're dating Virgin Branson Jr. Surely a guy like that won't feel threatened by some punk-ass kid such as yourself? Think again.