why did Rudy want excel at athletic competition?

I think there are a few reasons here. Rudy is naturally athletic. He also doesn't believer Nazi rhetoric. He admires the great Jesse Owens even painting his face black (out of tribute) and running around the track. Rudy also wants to win four track competitions and show up Franz Deutscher, the Hitler Youth leader.

He backed away from winning that fourth competition in deference to Jesse, I'm thinking.