Ollie and Dizzy were very good childhood friends, and in fact still are. It was Dizzy’s firm goal to teach Ollie how to fly, testing out his teaching skills to earn his cutie mark. Unfortunately their efforts were in vain, because of Ollie’s undersized wings. Dizzy didn't seem to understand this, and would continuously push Ollie off clouds and such to get him flying. It made Ollie a bit skittish around the other pegasus, though Dizzy was really well-intentioned. Despite this, they remain good friends and Dizzy likes to help Ollie strengthen his gliding skills.

Dizzy isn't necessarily stupid per se, he just needs to be firmly told by an authority figure not to do things like that to make sure it really gets in his head.