Gratitude and Robinson Crusoe

The theme at church this morning was being thankful.  I especially appreciated the part of my pastor's sermon about giving thanks amidst bad situations.  He referred to the book Robinson Crusoe, in which Robinson is stranded on an island and makes a list of good things and bad things in his life at the time.  A negative was that he lost his supplies, but a positive is that there was fresh fruit on the island.  A negative was that he didn't have many clothes, but a positive was that he didn't need them, since the island was warm. 

One can cynically think of reasons not to be thankful: how do we know that the good things in our life are truly from God, for what about those in the world who lack these good things?  Why doesn't God seem to provide for them?  And do we somehow have some of these good things on the backs of others?
 
I'm all for correcting social injustices.  But I also recognize the importance of being appreciative, especially since I tend to be a negative, glass-is-half-empty sort of person.