Value is purely subjective. Given that, it is an interesting question as to why someone would place value on an entity, and what the nature of that value is, though the scope of this query seems to be so wide that a simple answer is more prone to be facile than profound, e.g. claiming that what is valuable is identical to what is 'life enhancing' sounds vacuous and tautological, while claiming that it is that which helps us achieve our goals is at least slightly more specific, but it is also wrong, as we clearly value things that do not perform such a task.
Got to dig out my copy of Zen and the Art of Motorcycle Maintenance to try and understand again what Pirsig meant by 'quality.'