Full Idea
Only an agent's desires may constitute her having certain goals, and it follows from this that only her desires may constitute her motivating reasons.
Gist of Idea
Goals need desires, and so only desires can motivate us
Source
Michael Smith (The Moral Problem [1994], 4.8)
Book Reference
Smith,Michael: 'The Moral Problem' [Blackwell 1994], p.125
A Reaction
We might distinguish between reasons which direct us towards certain ends, and reasons which motivate us to pursue those ends. Most mornings I have a reason to get out of bed, which precedes my motivation to actually do it.