I have thought much about why this happens. Some of it assuredly is that I hate to leave any job undone. And the size and scope of these projects means that by definition there will be much work remaining.
There is also the natural let down when you leave friends (even ones you just made on the trip). Add to that the fact that upon leaving (and traveling) by the time home is reached, you have passed exhaustion. And of course there is the fact that when away, the world continued so you have a mailbox full of emails demanding answers and an already large backlog that has gotten even larger.
That said, I think the root cause is gloom is that the trip has made it abundantly clear that all is not well and that many people have it very tough path to travel down. How tough of path? Imagine losing your house and all of your possessions and in many cases your job. Now add to that sense of loss, your life itself is a jumble of insurance claims, waiting in lines, and errands to buy objects that only days before would have never crossed your mind. Can you imagine the feeling of despair and hopelessness? What makes that mental exercise more than just a mental game is that thousands of people are going through it right now.
Added together these items lead to a feeling of pessimism that briefly takes some of the luster off trip. However, in a short time the realization of that much good work was accomplished on the trip and that those same people whose losses helped cause the sense of loss in the first place, are now slightly better off because of you comes to dominate and the then upbeat volunteer will begin to think of ways to get back to help or minimally ways to help others at other sites but for now there is a short period of melancholy that must be worked through.
That and a large pile of dirty laundry. Off to the laundromat.