Natural rivalry

Draco Malfoy and Harry Potter hate each other, some times they are seen as rivals, some times like one is harassing the other in some way.

Essentially, they've never got along, and Ron Weasley himself, as well as Rubeus Hagrid and Madam Malkin all admit that since they met at both the train and the clothing store, it seemed like they were irritated with the other.

It's only escalated since.

When second year started, a prank war between the two students also began, and it hasn't let off since.

They sabotage each other's work when they came, they jinx the other, and even found a way to sneak into the other's common room to leave behind marauder-worthy traps.

When Harry found out Draco Malfoy became a snake animagus during fifth year, he decided he won't be one-upped by him, and though the month with the mandrake leaf on his mouth was hellish since he couldn't talk back to Umbridge, it was all worth it when he discovered his animagus form. A mongoose.

Afterwards, every time one succeeded in something, the other was close behind to succeed in it as well.