I don’t have much time, so I’ll let this speak for itself.  Perhaps not directly on point at first glance, but the heart of this Pensee is all about living in the present, which is at the heart of thankfulness.  It also highlights the effect and ultimate result of a life dominated by yearning for the future rather than contentment (thankfulness) for our present circumstances.

We do not rest satisfied with the present. We anticipate the future as too slow in coming, as if in order to hasten its course; or we recall the past, to stop its too rapid flight. So imprudent are we that we wander in the times which are not ours and do not think of the only one which belongs to us; and so idle are we that we dream of those times which are no more and thoughtlessly overlook that which alone exists. For the present is generally painful to us. We conceal it from our sight, because it troubles us; and, if it be delightful to us, we regret to see it pass away. We try to sustain it by the future and think of arranging matters which are not in our power, for a time which we have no certainty of reaching.

Let each one examine his thoughts, and he will find them all occupied with the past and the future. We scarcely ever think of the present; and if we think of it, it is only to take light from it to arrange the future. The present is never our end. The past and the present are our means; the future alone is our end. So we never live, but we hope to live; and, as we are always preparing to be happy, it is inevitable we should never be so.