You control whether an element requires start- and end-element tags in your element declaration via "O" (letter O as in "omissible") in the respective tag omission indicator position:

    <!ELEMENT e - -(f,g,h)    -- no tag omission -->
    <!ELEMENT f O - (#PCDATA) -- start-tag omission -->
    <!ELEMENT g - O (#PCDATA) -- end-tag omission -->
    <!ELEMENT h O O (#PCDATA) -- both start- and end-
                                 tag omission allowed-->

What's painful about end-element tag omission?

Can't find the source, but vaguely remember this line from a Usenet or mailing list post, maybe by Tim Bray:

The lack of </> in XML is a crime against humanity.