For example, the name Ericson comes from "son of Eric," so why did Eric's grandson take the name? At what point did the name Smith switch from being a title for a blacksmith to a common name? Someone over in /r/history mentioned the Dutch being forced to adopt a surname after Napoleon's invasion of the Netherlands, are there any other examples of a legal mandate like that?
Warning: The below dates might be slightly off. I'm working for memory here.
The 'why' is difficult. It's very hard to say why a byname (ie a nickname) suddenly becomes a formal surname. Particularly given that, in England, we're talking about a gradual and poorly documented process that occurred several hundred years ago.
What we can say though is the when. For England at least. By the time of the 1377-81 poll taxes it seems that most of the population had acquired a surname or byname, with only a minority relying on given name alone. This would have been quite different from two to three centuries previously. The caveats here being that the tax returns would have missed out on the poorer segments of the population and that there were significant regional variances.
So by the mid 15th C it's safe to say that most English people had a surname. But no one really knows why. The arrival of the Normans probably played a role* and there was a sharp reduction in the variety of English given names in the centuries after 1000. Did this necessitate the adoption of surnames or was it driven by the latter? I've not seen anything to conclusively prove it either way.
*And it's worth noting that across much of Europe the first centuries of the new millennium saw the concept of the family almost redefined for the landowning elite. Smaller immediate families became more important, as opposed to the larger kinship groups of the early middle ages, and we see the spread of primogeniture and the consolidation of ancestral family lands. It makes sense that these practices would later spread to the rest of the population.
(Incidentally, I don't know why this was necessary but I've seen references to Russian Tsars in the 19th C mandating that their Jewish subjects adopt surnames. I can only assume that, for some reason, Jewish communities in the Russian Empire did not use surnames.)
hi! I don't recall offhand that the adoption of surnames in England has been described before, but you'll find a few other examples in the FAQ