To the Democrats, the wall is an expensive, useless folly that sends a counterproductive message to the rest of the world that America is determined to keep foreigners and foreign ideas out. It is consequently an ugly metaphor for the entirety of the misbegotten Trump presidency. As an object, however, it is more silly than malignant. As a result, it can be accepted if it can be traded for something of real value, such as legal rights for DREAMERS. By elevating its importance, Trump has artificially increased its value as a bargaining chip, which should not be given away cheaply.
To Trump, the wall means solidarity with his base. Furthermore, it symbolizes the success or failure of his presidency, to himself as well as his more rabid followers. Since it was the centerpiece of his campaign, he has to have it, or what was the point of his candidacy? And he has to have it with few or no conditions, or he’s not a winner, which is one of the other principal selling points of his candidacy.
So how does this end? With a deal that Trump decides he can spin successfully as a “win” for himself and the wall. The substance of it barely matters; what actually matters is the symbol.