Sometimes a dream is short-lived by nature. Other times they fade because you change over time: your goals, your personality, etc.. If it changes by natural reasons then I say let it be.
However, if a dream changes simply because you run out of ambition then it’s nice to know that there is always more of that waiting for you to snatch it up. To do so I often have to put myself back in physical scenarios that remind me how important my dream is in the first place. So say you want to go to college and you are working really hard to get there. But you just run out of steam and are now thinking it’s too hard or it’s taking too much time. A good way to get yourself amped back up is to go to the local college and sneak in on a class. Or to go hear a speaker talk about how college changed their life.