Anybody whom we love we always want to be close to that person. A little absence of that person makes us shed tears for him/her. 'Miss you' means longing for one's presence. It is a sad emotion,a prolonged unfufilled desire to meet a person who has been very close but at present is very far. Sometimes communication gap also increases the distance between people which make them miss each other. If a person says 'I miss you' he/she means that he/she is thinking about 'you' and wants 'you' to be around. When the person is not with us we  recall the times spent with them,the things that we shared with that person makes us meet that someone.