ובקצרכם את קציר ארצכם לא תכלה פאת שדך בקצרך ולקט קצירך לא תלקט לעני ולגר תעזב אתם אני היי אלהיכם
And when you reap the harvest of your land, you shall not wholly reap the corner of the field, nor shall you gather the gleaning of your harvest; you shall leave them for the poor and for the stranger; I am the L-rd, your G-d. 
This mitzvah, included in the Torah-reading for Sukkot I & II, also appears in Sh’mot/Ex. 23:10-11, where it’s stated in connection with the sh’mittah (7th year), that the abundant produce of the field be left entirely for the poor.
In the present context, it’s mentioned within the annual cycle of holidays. The connection? G-d seems to be saying: “I took you, who were otherwise help-less, out from slavery and made you prosperous. As an ongoing reminder and expression of gratitude, leave part of your abundance for those who have less than you do.”
The Talmud further notes that Torah specifies no limit as to how much is to be left over:
“These are the things for which there is no limit: ‘Pei’ah’ [the corner of the field]…” 
Why should G-d place this mitzvah specifically within the mention of holidays that celebrate prosperity, and for the rabbis to further make no attempt to derive from Torah a limit to this generosity?
Because it often seems that the more we human beings have, the less we want to share.
For example, Millet’s painting (above) of “gleaners” (following the mitzvah in Torah) was severely criticized by 19th-century French Middle and Upper classes for portraying the poor in too “positive” a manner. It seemed to encourage “Socialism.” 
In these difficult times for the economies of America and the world, this mitzvah of “sharing” bears special remembrance. It also reverberates in the theme of the “Occupy Wall Street” demonstration currently in progress.
But it suggests another meaning, too: “Greed” can be addictive.
Just as the drug addict is less and less satisfied by greater and greater quantities of drugs, so can the person who bases their sense of security and self-worth on money become less and less satisfied by greater and greater acquisitions.
The more we have, the more we feel that we “need.”
“One who loves silver won’t be satisfied with silver…” 
When we see others who have more than we do, the feeling that we must not merely “catch up,” but actually surpass them, rages in us, as if we can’t really be “secure” until we have more than everyone else we see.
At the same time, we’re willing to heartlessly use those who have less than us, to acquire more for ourselves — much as an addict will steal, even from intimate family members, to meet the demands of his or her own addiction.
It’s no less true for the poor who become wealthy, than for those born to wealth.
G-d, then, seeks to free us from the corrosive effects on us and society of our “slavery” to addictive greed, by causing us to constantly remember that we haven’t succeeded purely as a result of our own efforts and requiring us (for our own benefit) to express (and feel) gratitude by sharing abundantly from what we’ve been given.
“Behold, this is the fast that I [G-d] consider precious: Let loose the chains of wickedness; undo the bonds of oppression; let the crushed go free; break all the yokes of tyranny! Share your food with the hungry; take the poor to your home. Clothe the naked when you see them; never turn from your fellow …If you remove from your midst the yoke of oppression, the finger of scorn and the speaking of malice; if you put forth your soul to the hungry and satisfy the wretched, then shall your Light rise in darkness and be bright as noon…”  
 Millet, Jean-François; The Gleaners (Des Glaneuses); oil painting, 1857
 VaYikrah/Lev. 23:22. Also part of parshah “Emor“
 Mishnah Pei’ah 1:1. The significance of including this as a reading in the traditional Morning service should be self-evident.
 Kohelet/Ecclesiastes 5:10. Also read on Sukkot.
 Yishiyahu/Isaiah 58:6-8; also part of the haftarah for Yom Kippur