Why do we have to write "I " and not " i " ? (for example : They thought I was dispappointed) Thank you!
I'm not sure there is a "reason", other than usage and convention.
In some very early texts, you do find a lower case "i" sometimes. Perhaps the original reason was to distinguish the first person singular from the numeral "1", which was often written "i" in early texts. (But that's only a guess!)
(I'd be interested to know whether any other members know more about the "majuscularity" of "I".)