Can I attack a Garruk Relentless that I control?

[Kerdo]

Can I attack a Garruk Relentless belonging to me?

[Arix]

No.

[Kerdo]

why not

[Arix]

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

[DatAsh]

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.

[Arix]

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.