When we are healthy, we don't always thank God for the gift of another day. We take it for granted and simply believe that we will keep on living.
Yet, every day is a gift to us and we do not know when the Lord will call us home.
That is why we should thank God each evening for being alive, thanking him for sustaining our life on earth one more day.
Here is a prayer from the Liturgy of the Hours that recognizes this fact and expresses gratitude to God at the end of a day.