Well, to begin with, proper nouns are specific nouns such as: names or places. They ALL require a capital letter. The reason for this is that, they are certain things like London Bridge or Taj Mahal or Luke. However, the word 'bridge' does not have a capital letter because it is not a certain bridge. This is then same for any other proper noun.
For a long time, English followed the model of the German language, which capitalizes all nouns. Some people also capitalized certain other words just to emphasize them. Many books from the 1600s and 1700s even have very chaotic capitalization decisions simply because the printer ran out of lower-case letters.
<span>Capitalization in English started to stabilize in the 19th century. Now, we capitalize proper nouns mostly to limit confusion. It simply gives us a way of clarifying what we're talking about. Check out these two sentences, for instance: </span> • "Newspapers report that the London bridge is falling down." • "Newspapers report that the London Bridge is falling down."
In the first sentence, the non-capitalization of "bridge" means that it could be referring to ANY bridge in London. The second sentence, however, clarifies that it is referring to a specific bridge whose name is the London Bridge.
In short: we capitalize proper nouns partly because of tradition, and partly to help us specify what we mean.
