Everyone has a memory of a time, a person, a place in their life that is wonderful. For them it was like living in their ideal world. In this life "all good things come to an end" as the saying goes. So when that good moment is gone, there is a sadness because their good experience has died. The person has suffered a loss and they are naturally grieving for it. From that point on, their life may not seem the same, without spark, only very ordinary, routine and dull. They remember the good times with sadness because they seem to have lost them forever.
The Bible teaches that God has placed eternity in our hearts (Ecclesiastes 3:10-12). We all know there is something better than this - when we have an amazing experience, we want to hold on to it forever but we cannot in this life. But what if these good moments in our lives were only a foretaste of the things to come? God uses these experiences to create a longing within us for what he has actually prepared ahead for us if we will believe and trust him.
These good memories in your life are merely a picture, a taste of all that God has planned for you (1 Corinthians 2:9). We therefore do not need to feel sad but can rejoice instead for what lies ahead. This is the promise that God has given, the Gospel that I have passed on to you.