Why is the pronoun "I" written with an uppercase letter, even when it's not at the beginning of a sentence? [duplicate]

This Wikipedia article says the following, about why I as pronoun of the first singular person is always written capitalized:

There is no known record of a definitive explanation from around the early period of this capitalisation practice.

It is likely that the capitalization was prompted and spread as a result of one or more of the following:

• Changes specifically in the pronunciation of letters (introduction of long vowel sounds in Middle English, etc.)
• Other linguistic considerations (demarcation of a single-letter word, setting apart a pronoun which is significantly different from others in English, etc.)
• Problems with legibility of the minuscule "i"
• Sociolinguistic factors (establishment of English as the official language, solidification of English identity, etc.)

This happens only with I; other pronouns (including me) are written as any other words, and capitalized when other words would be capitalized.

As @Bill Franke stated in the comment, it's a general convention, followed by anyone willing to write proper English.

All other pronouns (such as you, he, she, ...) should not be written with uppercase letters, except - of course - when they're at the beginning of a sentence.

For the remainder of nouns, it depends on what it is. Generally, only people's names (e.g. Jeff, Nicole) and places (e.g. Italy, Rome, ...) are capitalized. But, of course, there are - like in every other language - exceptions to the rule.