I'll take a crack at this:
That's a way of looking at it that isn't necessarily incorrect. Quite literally, if I am depressed, but there are good things in life that have every reason to make me happy, and I am still depressed anyways, that would require some lack of appreciation or gratefulness of those things on an emotional level. The same would go for boredom if there is entertainment, or excessive worry when a perspective shift would have me look at things differently.
The problem with viewing depression as mere ungratefulness, though, is that there must be a cause for this ungratefulness that is not present in the normal person. Something is causing it to happen, and even if the depressed person is "ungrateful" that doesn't solve anything or really get us a solution. The chemical makeup of the brain is causing the problem, so it's unlikely that telling depressed people to not be depressed and be grateful instead will help.
Maybe it could help some. Maybe being confronted with this will cause a perspective shift which could set someone down a better path, but it's not likely.
