In one of my recent games, I played shaco and was praised because I did well

Then, I played another game right after and was matched up with the same guy that praised me from the last game. I was forced to go Riven adc, so I didn't really do that well. The same guy that praised me kept flaming at me and saying I was terrible and that I was trash tier. I didn't realize that your score in a single game made you good or bad.