The Mitzva of השבת אבידה, returning a lost article, was mentioned in פרשת כי תצא. We learn from this Mitzva, the concept of לפנים משורת הדין, going beyond the letter of the law. There are times when we might be allowed to keep the lost article, but we are taught to do more than the strict letter of the law.
This is meant to manifest itself in other ways that we interact with people and the Mitzvot. We are not to be satisfied with only fulfilling our obligation, but we should try to do even more than what is expected.
Another aspect of השבת אבידה is taught by the ספר החינוך. He mentions that שכחה, forgetfulness is a very normal human trait. Therefore, when someone returns something we lost, there is great joy and relief. The Gemara gives an example of this regarding lost תפילין. If someone finds such תפילין, he is allowed to return them on Yom Tov. The rabbinic prohibition of מוקצה is overridden by the joy that is brought to the one missing his תפילין. So we see that השבת אבידה is an important Mitzva, on many levels.