This is because the verb s'habiller is a pronominal verb (je me habille), so they take être for the passé composé (and the participle has to agree with the gender and plurality of the noun).

Uhm, thank you.

Because you dressed YOURSELF. If you had dressed SOMETHING OR SOMEONE ELSE it would be avoir. Ex : J'ai habillé les enfants. There are several verbs like this suing both auxiliaries. Like "sortir" Ex: j'AI sorti la poubelle. je SUIS sorti(e).