Sportsbooks charge vig (sometimes called “juice”) on every bet, helping them make money. The odds are always on their side, so you can’t just win 50% of your bets to break even.
To calculate the exact percentage of bets you must win, we have to find the implied probability. You can learn about reading betting odds here and calculating implied probability here. For standard -110 lines, as an example, you have to win 52.38% to break even.
If the percentage of your winning bets is higher than this number, you will be making a profit. While the added 2.38% doesn’t seem like much, it is a hurdle that most sports bettors fail to cross.