Hume's argument is that all preferences and motives are emotional. There is no such thing as an unemotional or purely rational decision, because to decide, by its nature, is to have a preference for, i.e. an inclination toward or aversion to, something.
Reason's (i.e., cognition's) role is to structure the world for us: it lays out a schematic of how objects and ideas are connected. But this is merely cold information, devoid of any significance on its own. Rational categories have no value or priority without feeling.
The passions (i.e., emotions or affects) are necessary to evaluate any object or idea as valuable, problematic, virtuous, immoral, good, or bad, because these forms of evaluation are all based on either a positive or negative impression. Decisionmaking is neither an rational nor "irrational" (whatever the hell that means) process: it is based on preferences, which must arise from emotional states and can never just magically be produced by reason alone.
What reason does is enable things to become positive or negative by association. I have a goal that I care about, and reason suggests to me that a previously unimportant object will help me achieve it. Thus the object itself becomes important to me. Reason creates the association--it's a telescope that allows me to see the distant or indirect emotional consequences of the object--but the emotion itself remains my only motive and only decider.