When you write 1 and 2 one below the other, copy them and drag it down the numbers increase calculating the difference between the first two cells. However you can’t do the same using alphabets because it just copies them down.
There is a workaround on this, if you want to add Capital letters Type 65 and under that 66 and drag it down to 90, then type = char(cell) and drag it down. For small letter you need numbers from 97 to 122. You can use CHAR function for many other things, so i’ll write a separate post for that.