Modesty is at one end of a scale of which the other is boasting; generally the former is considered polite while the latter is definetly not. But why is this?
If we look at the middle of that scale, we have an (our percieved) accurate measure of our abilites. Logically speaking, it should be this that is looked on well; by providing a good estimation of our abilites others then will know whether we can be relied on; for example, overhyping one’s skill at playing football can cause your team to fail, while downplaying your skill might lead to resenting having not picked you to play. Honesty about one’s abilities seems to be the logical approach.
But of course, emotions aren’t logical. Looking at it from an evolutionary point of view, would modesty be valued? The male is expected to be strong, virile, and have access to resources; all of which suggest that boasting, in fact, would be the best course of action to take (and indeed, many people do present themselves slightly better than they are when dating). Again, for females, fertility and a lack of promiscuity are areas which can be boasted with. If we take this view, then boasting should be seen as the norm - and so accepted - and certainly not modesty.
So where does the preference for modesty come from? It seems likely it’s a purely social concept; but from where did the motivation for modesty come from? What advantages does it have, beyond satisfying our own and other’s need for modesty? It’s a circular relationship; what started the circle?