Can I attack a Garruk Relentless that I control?

Can I attack a Garruk Relentless belonging to me?

No.

why not

Same reason you can't discard cards whenever you want. You just...can't.

If somebody like Arix says you can't do something in Magic, 100% of the time it's because the game rules say you can't.

Keep in mind that the rules of Magic are permissive, not restrictive. That means they don't tell you what you can't do - they tell you what you can do, and you can't do anything else. Basically, if there isn't a rule that explicitly lets you do something, it means you can't do it.