Yoda explains that the cave has "only what you take with you." The cave is a reflection of your own psyche. Luke is realizing that he has much more in common with Darth Vader than he dares admit (he is at heart a passionate, angry, petulant and cynical man), and the way Yoda and Obi-Wan treat him could also be tipping him off that something's wrong (how strange Yoda would be so reluctant to train the son of a great Jedi, etc.). The vision in the cave is a manifestation of his doubts. I am just like Darth Vader. Yoda and Ben seem to know this. Why is that...?

Later Darth Vader tells him to "search his feelings" to know that he is truly his father: deep down, Luke knows that it only makes more sense that his father were more like him than the rosy, idealized image that Obi-Wan had sold him. It's all an allegory for growing up and realizing that your parents are just as dumb as you are, lol.