Why are Mario and Sonic the Hedgehog rivals

In the begining when Sonic did not exist, Mario games and Nintendo dominated the gaming world. SEGA decided the needed a mascot too. Then Sonic the Hedgehog was born. Sonic and Mario games fought furiously until SEGA over came Nintendo. Now they both are at level. They now work together to make the legendary Mario And Sonic games. The rivalry is just a remembarance and a appreciation of those days.

Sega never overcame Nintendo; Sega just gave up and stopped making video game hardware.