I've recently noticed something along these lines, when I get something i want i frequently get a quick twinge of anxiety or an unrelated worry flashes in my mind suddenly. It feels like I mistrust good things happening, like the 'other shoe' is bound to drop if I allow myself to feel simply contented about the good bit of fortune. That's the anecdote but I also listened to a dharma talk recently saying excitement (under the subheading of desire) carries inside it a sense of lack, as xyingyangx says above and teases out further. Remind yourself to turn toward this feeling next time to investigate for yourself, as will I.