Two words with nothing attached. No object, no explanation, no list.
Most gratitude comes with an argument built in, thankful for this because of that, and an argument can be won or lost. Someone can point out that your job is difficult or your family complicated, and the case falls apart.
This one has no case to lose. It is less a conclusion than a posture, standing slightly open toward the day before you have decided what the day contains.
You can hold it for four seconds on a difficult morning and it still counts. Nobody is checking the reasons.