why is it correct to use discuss rather than discusses in some sentences
It depends on the subject. If the subject is 3rd person, singular (noun or pronoun), then the present tense verb form is "discusses." If the subject is anything else, singular or plural, noun or pronoun, then the verb form is "discuss."
I discuss...
you discuss ...
he, she, it (or any singular noun) discusses...
we discuss...
you (all) discuss...
they discuss...
http://grammar.ccc.commnet.edu/grammar/sv_agr.htm
In all the explanations and examples here, notice that all the present tense, 3rd person singular verbs have the -s ending; all others don't.