‘Sunrise doesn't last all morning. A cloudburst doesn't last all day.’ – George Harrison
Everything comes into and leaves this world as the result of a combination of causes and conditions. Your thoughts come and go. Rivers run fast then slowly. You are born then you pass away. Nothing is permanent and all eventually ends.
Mindfulness helps you to understand this. You can appreciate the good, the enjoyable and happy times. You will be more able to make the most of now, knowing it will not last.
You will also know that suffering, problems and difficulties will not last. One way or another, they will pass.
Mindfulness helps you to recognize the futility of hanging on or getting attached to thoughts, feelings, ideas, people and events. They will pass anyway.