It's all about how the individual finds the motivation needed. Some its money, some its fame. It can be hatred, seeking new records to break ect ect. Where all different and do what works for us.
A good point - I can't remember which rider it was (Fogarty?) who said he had to work up a hatred of the other guy, much like some boxers need to.
IIRC, Sheene vs Roberts was always acrimonious.
Ago vs Hailwood vs Read were always gentlemanly affairs... but there was intense rivalry.
IMO, it isn't always about a shitfight - it is just competition. Like McEnroe and Borg - intense on court, hanging out afterwards.