What is a perfectly good reason to run away?
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I personally think any reason is good enough. If you want to, go for it. There's nothing immoral about running away. If your situation between you and your parents is bad enough that you want to run away, then you don't deserve to be in that situation.
I know many people would want to respond, "well sometimes the kid just doesn't respect rules and limitations or what's best for them etc etc etc." but I don't buy it. It's unnatural to have to have your life dictated to you by someone else, and the vast majority of parents don't really have the wisdom to make good parents. Wise parents don't foster/allow an adversarial relationship between them and their children.
People weren't even meant to function in a nuclear family scenario anyway; children were meant to be raised by the community. Granted, we probably don't really have the kind of communities nowadays to make that practical. We could at least have larger nuclear families though. (I say "probably" because it's hard to say how much fear of strangers is being overly paranoid and how much is legitimate.. 30 years ago parents could drop their children off at a park and leave and not get arrested for it, and abduction, etc. rates weren't any higher..).
The demands we put on kids, such as not having sex, going to school, doing your howework, studying, not going out at certain times, etc. are also unnatural. It's not always the parents' fault, some of that is the fault of the society the parents and children find themselves in, but that doesn't mean that running away from it necessarily a bad decision, much less that it's immoral.