Question about stocks

EDIT: Thanks for the information, everyone. I'll be sure to tell him to do more research. <br><br>---<br><br>I have a quick question. I recently invested a bit of money in the stock market through vanguard and the total value of my stock went up. If I were to cash out now, the total amount of money I would get would be more than what I originally invested. I have a friend who recently invested in the stock market, he invested for a shorter amount of time than I did and his stock went up by a higher percentage than mine did. He's going to be able to cash out with a larger profit than I will, even though he invested for less and spent less than I did. He's concluding from his observation that since he's doing better than someone who invested more money and for a longer amount of time than he did, then he will always be able to make more money investing than I can. I feel like his reasoning is flawed. What would be a good way to respond?