He's a playboy and his friend bet him a thousand dollars if he could make you fall in love with him. So he started flirting with you and you showed him that you weren't interested in him

As he looked at you, he realized he was truly falling in love with you. He began to wonder why you were the only girl to reject him in spite of the fact that other girls would kill to be in your position, as he whispered to himself

"This was no longer a game, im falling for her."