In Parshat Lech Lecha, G-d first promises Avraham that his seed will be as numerous as the dust of the earth and then as the stars in the sky. These two promises are a contradiction in contrasts, dust is low and common and stars are great and glorious. What do such promises mean?
Dust is the source of human life as man comes from dust and returns to dust leaving behind nothing but dust. By contrast the light of stars endures even after the stars themselves are gone, their light shines on to distant places. Since these promises do not apply to merely one generation but the entire continuity of the Jewish people, we might consider them to be two ways of looking at what a generation of the Jewish people will leave behind after it passes on.
A generation that is dust may be numerous and cover the world but what it leaves behind is mere dust. A generation that is like stars may die but the light of their greatness shines on after them in this world. It also reflects two ways of looking at those who are dead. To the simple the dead are like dust whose passing leaves nothing but dust, to the wise the dead have left behind their knowledge which like light shines into our lives.
Why these two different promises then made to Avraham? Unlike the previous two Parshas, Bereishis and Noach, both of which begin on a high note, the creation of the world with man at its pinnacle and Noach as the perfect Tzaddik salvaged from the flood to begin a new world and end in ruin and decay as Adam's children murder one another and go on to corrupt the world and Noach's children repeat all the mistakes of the past and create a tower to fight against G-d himself; Lech Lecha begins at a low point with Avraham told to leave his home and to wander, remains childless, faces war, domestic turmoil and uncertainty over his future ending in the high note of the promise of the birth of Yitzchak who will continue his righteous line. Where in Bereishis the creation of the heavens precedes the creation of 'dust,' with Avraham the order is reversed with an ascent of accomplishments, rather than a descent of accomplishments as occurs throughout most of human history.
Both of the promises are tied to specific events or rather a specific person, Lot. The first explicitly so, when the Torah makes a point that G-d speaks to Avraham after he parts from Lot, as if parting from Lot in some way enabled G-d to speak to Avraham. After this separation God promises Avraham the inheritance of the land and his descendants to become as numerous as the dust. This is the promise of a physical inheritance, an inheritance of earth, a growth of physical numbers.
When G-d makes his second promise to Avraham it is after he has rescued Lot from the kings and turned down any offer of a reward that G-d promises that his descendants will be as numerous as the stars. This is not merely a promise but a covenant sealed with sacrifices and the promise of a spiritual destiny, rather than merely a physical one. G-d has already promised Avraham that his children will inherit the land, the new element is their slavery in Egypt until then. As a reward this seems more of a punishment, unless we understand that this is a trial, for Avraham's descendants who will inherit not merely land or wealth but his spiritual destiny. As Avraham did in Lech Lecha, they will begin at a low point to be elevated and taken out of slavery to be G-d's people. From the dust, they will ascend to become stars.
But what is the relevance of Lot in all this? Lot is not an evil man but he chooses greed over godliness. The first time Avraham parts with him, it is over grazing rights, essentially property rights disputes. Lot chooses wealth even if it comes at the cost of wickedness. Avraham chooses to follow G-d even if means danger and impoverishment. The reward Avraham receives is a promise that his descendants will gain the wealth and property that he passed up, becoming as numerous as dust, an aspect of the earth, and inheriting the land.
When Lot is captured by the enemy kings, Avraham marches into battle to save him and gives up all the property refusing to be enriched by it. This time when Avraham leaves Lot behind the dispute is not possibly over property. Previously it might have been said the two had parted ways because Avraham would not share his wealth but now it was apparent that Avraham refused even the wealth he had coming to him. His separation from Lot this time was a spiritual separation and merited a spiritual reward.
What was the importance of separating from Lot? The generations before the flood became corrupted by intermingling with the children of Kayin. The Jews would become corrupted by mingling with Caananites, Philistines, Phoenicians, Greeks, etc. Time and time again the children of Avraham would need to divide themselves from those who would corrupt them, Yishmael and Esav and Lavan. Lot was ultimately the first test. By first dividing himself from Lot the first time, Avraham was rewarded with the promise that if his descendants could divide themselves physically from those who would corrupt them, they would achieve material prosperity. The second time he was promised that if they could separate themselves spiritually from them, they would achieve spiritual prosperity.
If they could separate themselves both spiritually and physically, survive the trials and the wanderings than the Jews would achieve the level of greatness that would allow us to be numbered among the stars whose light shines on after them.