Some incidents, moments, and people are difficult to forget. Time goes on and we get stuck in those moments where we feel we have lived our life. We find happiness in remembering that time.
"Those thoughts" keep our minds so busy that we neglect what's there in our present. We focus on those moments so much that we feel that was our time, that was our life, and often forget that it is a past that has gone and no matter what you do you won't get it back.