The problem with time is that it is finite. There are the same number of seconds, minutes, and hours in every day. What this means is that we have to fill up those seconds, minutes, and hours with something. And some of it boils down to simple math. Let's take a look.
There are those people that we all know who live very full lives. They have very packed days. Their days probably require a great deal of structure to keep it all together. They may change topics ten to twenty times a day. They go to work. They go to lunch. They talk to co-workers. They run an errand. They have a doctor's appointment. They go to the gym. They meet a friend for dinner after work. They do some of their laundry. They read a book. They have a bedtime routine. They go to bed. Their days may be so packed, in fact, they may even forget a couple of things on their list or find there are still things they didn't get to.
There are also those people who have much smaller worlds. Some people have a lot less things to do. They may work less. They may no have as many friends. They may not have errands to run or choose not to do them. They don't workout. They don't have plans after work. They may or may not do their laundry. They may not change topics many times throughout the day. Whatever it looks like, I think you get my drift. The point is they fill a lot less seconds, minutes, and hours of their day. But... these days needs to be filled.
If our thoughts are constantly moving from one thing to the next we may not spend very much time thinking about any one thing. Filling on average sixteen waking hours with sixteen different topics can keep our minds bouncing. While there are plenty arguments to suggest a day can be too full, look closer at the opposite. If the opposite is an isolated life it leaves a lot of time to spend on a very few subjects. It leaves a lot of room to take a small thought and make it very, very big. Take a minute to sprinkle in a little extra dash of OCD and you've got an entire day that can be filled with a nothing problem gone haywire.
Depression and other related issues, such as anxiety, can cause the desire to shrink away from things in life leaving a lot of hours to fill. Structure and adding things to your life can help combat these ever growing thoughts. The structure shrinks the amount of time one thought can ruminate and can keep it contained. If you leave the space open ruminating creates an illusion of a full life. There key is paying attention to the difference between when a life is full and healthy or full and destructive.