Why is "I" always written with a capital letter?

The first person subject pronoun, "I", is always written in uppercase (majuscule). In French and most other languages it is written just like all the other pronouns. I spent ages this morning (it's my day off) trying to find a decent explanation, without much success. The most common theory is that when the old English "Ich" (like in German) became reduced to "I" it was thought to be too small and insignificant to be a real word and could easily get attached to the end or beginning of another word. So scribes and later printers took the habit of capitalising it. If you have another explanation, let me know. In emails and text messages the lowercase "i" is now becoming common. Personally, i don't approve!