Reason is not motive for human actions.

We always want to justificate everything we do, we want to give it a reason, a "why?" if we don't, we feel like what we are doing is not valuable. We don't notice that everything we do, even asking the word "why do this?" comes from emotions and impulses that can't be justificated with other answer than "because I want it" or "because it hurts" or "because I feel". Everything that everybody does, comes from emotional will. This is why Morals can't be justificated with reason, we can always give an "appropiate" answer to the question "why we want to do it?" but not a certain one.

David Hume in An Enquiry Concerning the Principles of Morals explains:

"It appears evident, that the ultimate ends of human actions can never, in any case, be accounted for by reason, but recommend themselves entirely to the sentiments and affections of mankind, without any dependance on the intellectual faculties. Ask a man, why he uses exercise; he will answer, because he desires to keep his health. If you then enquire, why he desires health, he will readily reply, because sickness is painful. If you push your enquiries farther, and desire a reason, why he hates pain, it is impossible he can ever give any. This is an ultimate end, and is never referred to any other object.
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Thus the distinct boundaries and offices of reason and of taste are easily ascertained. The former conveys the knowledge of truth and falsehood: The latter gives the sentiment of beauty and deformity, vice and virtue. The one discovers objects, as they really stand in nature, without addition or diminution: The other has a productive faculty, and gilding or staining all natural objects with the colours, borrowed from internal sentiment, raises, in a manner, a new creation. Reason, being cool and disengaged, is no motive to action, and directs only the impulse received from appetite or inclination, by showing us the means of attaining happiness or avoiding misery. Taste, as it gives pleasure or pain, and thereby constitutes happiness or misery, becomes a motive to action, and is the first spring or impulse to desire and volition. From circumstances and relations, known or supposed, the former leads us to the discovery of the concealed and unknown: After all circumstances and relations are laid before us, the latter makes us feel from the whole a new sentiment of blame or approbation. The standard of the one, being founded on the nature of things, is eternal and inflexible, even by the will of the Supreme Being: The standard of the other, arising from the internal frame and constitution of animals, is ultimately derived from that Supreme Will, which bestowed on each being its peculiar nature, and arranged the several classes and orders of existence."