Luck is like a prostitute,almost everyone has it for a while.
It cannot be always with you,you cannot control it,you cannot rely on luck.
In my point of view good luck or bad luck it's an event which occur against the odds,an event which is rare but happens,if it's good event,then we call it good luck,if it's bad,we call it bad luck.
You wouldn't call luck something which happens everyday or very often,like traffic for example.
Therefore my definition about luck=rare events,regardless if it is positive or negative.