What is the brain doing?

I had such a wonderful day with my husband and daughter today. We spent the day shopping and eating out. I got me some new cute boots. Had good food. And was happy with my food choices. I loved today. So why is it when I have happy days like this my brain says, “Yea, but you know it doesn’t last. Tomorrow will probably be hard and definitely not as good as today because every day can’t be this great.” What the heck is that? Is this normal and healthy or do I need some brain management on this one?