I answered a similar question here, although it covers more of the initial abolition of slavery than the 1802 reinstitution. This is all very interconnected with the Haitian Revolution, so I'll focus on that.

First, let's do a bit of stage-setting. When Napoleon took power in 1799, one of his pet projects was the revitalization of an overseas French Empire. He had grand plans for French Louisiana and the formerly prosperous colonies in the Caribbean, although Saint Domingue (modern-day Haiti) presented him with a problem. The colony was under the direct control of Toussaint Louverture, a former slave turned revolutionary who had spend the better part of the last decade fighting alongside the French after the first abolition of slavery in 1794. Ever since Napoleon first took power, relations between the two had been...strained. For some specifics, let's turn to Jeremy Popkin's A Concise History of the Haitian Revolution. At first Napoleon issued warm and fuzzy reassurances that, "the sacred principles of the liberty and equality of the blacks will never be attacked or modified" (105). Toussaint wasn't buying it, and noted that, "We are free today because we are the strongest. [Napoleon] maintains slavery in Martinique and the Ile Bourbon: we will also be enslaved when he becomes the strongest" (ibid).

Toussaint was right, slavery was still rife in other parts of the French Empire, despite the emancipation decree years earlier. Napoleon always had a prejudiced view of blacks, with Popkin quoting him as saying, “I am for the whites because I am white; I don’t have any other reason, and that one is good enough. How could anyone have granted freedom to Africans, to men who didn’t have any civilization, who didn’t even know what a colony was, or what France was?” (115–116). What a charmer. However, Napoleon couldn’t spare the men, time, or ships to change the situation in Saint Domingue, so he resorted to friendly diplomatic overtures to try and secure Toussaint’s allegiance. As time went on, Toussaint became more and more steadfast in his refusal of any French gestures of cooperation that he viewed as insincere. Napoleon, however, desperately wanted to recapture the former glory of Saint Domingue. The island used to be the crown jewel of France's colonies, and the revenues from it's sugar production made it one of, if not the, most valuable colony in the world, but years of war and revolt had left it in a poor economic state. Napoleon saw the restoration of Saint Domingue to its former glory as the key to developing his overseas empire. When it was clear Toussaint would not let this happen at the expense of the freedom of the blacks, Napoleon turned to reinstituting it by force.

It wasn’t that simple, though. Napoleon had more pressing concerns elsewhere, and needed an opportunity to strike at Toussaint and Saint Domingue. That opportunity wouldn’t come until 1802. By then, Toussaint had established himself as the head of a nearly autonomous Saint Domingue, a head Napoleon hoped could be cut-off and replaced with someone more loyal. Toussaint had just defeated another rival ruler named Andre Rigaud in The War of Knives, in which Rigaud had tried to reinstate a race-based hierarchy, with mixed-race peoples (like himself) above blacks. Toussaint rejected this, and Rigaud was forced to flee to France. (“Hey, that guy might make a good replacement for Toussaint!” thought Napoleon). Toussaint also expelled Léger-Félicité Sonthonax, a white French diplomat who proclaimed the initial emancipation decree and secured Toussaint’s allegiance to France, which left Toussaint free to do just about whatever he wanted, free from direct French influence. Toussaint even led a military occupation and annexation of the Spanish half of the island (modern day Dominican Republic) just to shore up his position. From Napoleon’s perspective, Toussaint had done all of the hard work of consolidating power on the island and restarting the plantation economy. Napoleon hoped he could switch Toussaint out for someone more compliant without so much as missing a sugar cane harvest.

It wasn’t until Toussaint promulgated his very own constitution without any consultation from the French that Napoleon felt things were getting a bit out of hand. It was around the same time Napoleon saw opportunities lining up. Whereas earlier most of the prominent French politicians had been pro-emancipation, the War of the Knives and Toussant’s other actions besides had split opinions, and Napoleon felt he could now find enough support for an invasion he always wanted. Even better, Napoleon secured peace with Britain in late 1801. This meant Napoleon could sail an army to Saint Domingue without the mighty British fleet (literally) sinking their efforts. And sail them he did. The LeClerc Expedition set sail with 20,000 men and a smattering of Haitian political dissidents (including Rigaud). Landing in Saint Domingue, the situation soon devolved into a frantic guerilla war with the French army suffering enormous casualties from fighting and disease, eventually forcing them to retreat from the island, but not before capturing Toussaint, who would go on to die in a French prison in 1803.

With the key to his new French empire lost, Napoleon abandoned that particular ambition, selling French Louisiana to the fledgling United States and focusing his efforts instead on newly resumed wars in Europe.

Edit: for spelling and formatting, as always.