Does running ever actually become enjoyable, or do people just get used to it?

Both, in a specific order. The first six to eight weeks are genuinely unpleasant for most people because your aerobic base is not built yet and every run sits near your limit. What changes is not willpower but physiology — once easy running becomes genuinely easy, the experience is different. The most common mistake is running every session too fast, which keeps you permanently in the unpleasant zone. Running at a pace where you can hold a conversation feels absurdly slow at first and is the thing that eventually makes it enjoyable.