What is the meaning of When one door closes another door opens but you so often look so long and so regretfully upon the closed door that you do not see the ones which open for us?

When people say that, they are trying to offer encourage and hope when someone has faced the end of a hope, dream or situation (closed door). For instance, someone might say it when offering encouragement to a person who has been turned down for a job or to someone who has been dumped by a girlfriend or boyfriend, letting them know that something better is still out there.