Having spent his life until his teens stealing from food courts and off-licenses, getting to be a superhero alongside the rest of the Teen Titans was more than a bit of shock to the young Roy Harper.
Used to enjoying life where and when he could, Green Arrow's apprentice took to party life straight away, and embraced it so much that it began to genuinely effect his prowess as a superhero.
Enraged by this, Oliver Queen kicked Roy out, the stress of which would convince Harper to become a drug addict. While Queen was horrified to learn what had become of his sidekick, it would be sometime before the two would actually reconcile - and even when they did, Roy would refuse to become his sidekick again, venturing out for himself instead under the identities of Arsenal and later Red Arrow.
Oliver wasn't exactly the most supportive mentor, however, as it was actually a pep talk from Killer Croc that helped Harper get his life back on track. When a giant evil crocodile is doing a better job at being a guardian than you, you might need to have a think about your life.