Rav Schwab says that the word Modeh means to admit. When we thank Hashem, we admit that what we thank him for comes from him.
When a person is given something that he likes, the automatic, animalistic response is, "more!" This causes the person to turn toward the source of this good stuff. This is how animals are trained. We give dogs treats, and they want more, so they try to figure out what made them get the treat.
If we had everything we needed or wanted, our automatic nature would over power us, and we would be totally self-absorbed. Hashem creates us with lacks, so that even by our animalistic nature we turn outside ourselves.
The next step, for higher beings, is to desire to give back. This can either be motivated by a desire to return to a state of self-absorption, or self-sufficiency, (by getting even), or it can be motivated by a desire to engage in the give and take of a relationship. Either way, a relationship is thereby established, which is the only way a person can be whole or complete (ie. by being other oriented, and not being alone, and allowing others into their lives.)
Perhaps this idea can be found in the blessing: Borei Nefashos Rabos V'Chesronan, Al kol ma shebarasa lehachayos bahem nefesh Kol chai. Baruch Chei Ha olamim.
Thank you for starting this beautiful thread.