Appreciate Someone

Appreciate is the word that a lot of people does not understand until they lost it . Example : A guys love someone then the guy will chase that girl no matter what it cause , before the guy success the guy will appreciate that girl but once the girl accept his love the guy will forget about the exist of the girl ... Then once they break up , the guy will start appreciate the girl back . Why people won't appreciate someone when they have it and let it gone ... 

What does a APPRECIATE mean to you all ?