If you really love a certain sport, it would be easy for you to understand things about the said sport. How most players would play, teams that are most likely to win and such. Let's say you've been a fan of basketball ever since, then you would ways watch games. It's easy for you to know what team is stronger. So if you would make a bet, you know who's most likely to win.
It's just a matter of knowledge about the sports and the players. Unlike other gambling who need luck, sports betting is more of analysis based on their history of performance.