Hume argues that moral norms are derived from experience, custom, and tradition. We act the way we do through habit and not on the basis of rational calculation. To be sure, reason does have a role to play in helping us achieve our moral goals. But the initial impulse for behaving in ways that we recognize as moral comes not from reason, but from the emotions.
As Hume famously states, reason is, and ought to be, the slave of the passions. This doesn't mean that reason is unimportant when it comes to morality. It simply means that reason alone doesn't move one to act. Only the passions—broadly speaking, the emotions—can do that. The initial force that propels us to act in the way that we do—say, love, anger, hate, envy, or fear—is emotional.
Once we're so motivated to act, then our reasoning faculties come into the picture. They have a very important part to play in determining which actions to perform and why. In this regard, it is reason that analyzes facts, perceives relations between objects, and draws conclusions. In short, it is reason that provides the rationale for our moral actions, but the initial impetus can only be given to use by the passions.
To extend Hume's master–slave metaphor further, one could say that the master—the emotions—comes up with the plan, but relies on his slaves—the reason—to help him put his plan into practice.