why: just because
the 'me' is called a reflexive pronoun or complement of direct object
there are verbs called pronominal verbs.
you must use a reflexive pronoun with these verbs (ie: 'me')
you also have te, se, nous, vous
examples of pronomial verbs:
je me couche / je m'endors
je me brosse les dents
je m'approche de qqn
je me rend compte / i'm aware
je m'en doute / I knew it
There's a whole ton of verbs like these.
Remember that when putting pronominal verbs in passé composé, you must use
être :
je me suis approché de lui
on s'est approché de lui
these reflexive pronouns can also be used as c.o.d (complement of direct object) or c.o.i (complement of indirect object)
examples:
je t'aime.
qui m'a volé de l'argent
il m'indique la marche à suivre