Rabbi Naamah Kelman:
The offering of thanksgiving in the form of sacrifice, or later in our tradition in the form of prayer, is the highest expression of gratitude. We praise and we exalt and recognize the miracles of our lives. Deeper gratitude recognizes the simple gift of our lives and the gifts of our daily lives. Our rabbis, ancient and contemporary, are telling us that perhaps this is the practice we must really cultivate: the practice of thanksgiving! But how?
Rabbi Doug Zelden:
There is a beautiful explanation about when the chazzan repeats the Amida aloud and says the Modim prayer and the congregation recites softly a prayer known as 'The Rabbis' Modim.' Why is that? It is because the cantor can recite aloud all the blessings in the Amida and be our agent for such prayers as 'Forgive us,' 'Heal Us,' 'Bless Us with a Good Year,' and so forth. With all our pleas, the cantor or Baal Tfillah can be our public messenger and say the blessing for us, as we answer Amen.
However, there is one thing that no else one can say for us. We must say it for ourselves. That one thing is 'Thank You.' Hoda'ah has to come from ourselves. No one can be our agent to say Thank You.
That is the reason for the 'Rabbis' Modim.'