An ex-lover appearing in a dream reflects lingering feelings or current emotional state. Being happy with an ex suggests you currently need warmth in relationships. Arguing with an ex shows past wounds still remain. This dream usually reflects current loneliness rather than a desire to return to the past.
Relationship-centered dreams such as "Ex-lover Appearing" often reflect feelings about a current relationship or something left unresolved. Freud saw these as an outlet for attachment or conflict suppressed during waking life, while Jung suggested the other person in the dream can also be a projection of a part of yourself.
Is there an unresolved feeling in your relationship with that person?