Why is I capitalized in English?

Old and Middle English used forms like ic and ich, which shortened over time to a single letter. That created a practical problem for scribes: a solitary lowercase i, especially before the dot became standard, was a faint vertical stroke easily lost among neighboring letters or mistaken for part of another word. Enlarging it to a capital made it visible and unambiguous. The change appears gradually in manuscripts from around the 13th and 14th centuries, becoming consistent in printed English thereafter, with Chaucer's era representing part of the transition.

The important point is that the reason was typographic rather than egotistical. English writers are periodically accused of capitalizing the first person out of cultural self-regard, and the timing does not support it, since the practice predates the individualism it supposedly expresses and was driven by legibility. English is nonetheless unusual here: German capitalizes the formal you, Sie, and all nouns, while leaving ich lowercase; French, Spanish, and Italian leave the first person lowercase. English also leaves every other pronoun lowercase, which is the detail that undercuts the vanity theory, since a genuinely self-regarding orthography would presumably not have stopped at one word.