This is a false dichotomy which falls apart under a little introspection. In attempts to contrast reason with emotion, we are confusing the message with its medium.
We are biological entities, and we refer almost every aspect of our state of mind through our senses and emotions. Even when you are acting logically, that logic is mediated through an emotion. We have a feeling of 'rightness' or 'clarity' when we accept our own deductions, which is, in itself, a physiological sensation that accompanies or predicts the mind's ease with the decision. If we cannot reach that point in some way, we do not find the action reasonable, whatever its source.
We rely upon our emotional machinery to accommodate our experience and to come to give this signal of 'rightness' in the presence of appropriately logical deductions. But we expect the same of our other emotions when we learn to fear the sound of an oncoming car but not to be afraid of the dark. Effective fear that has served us appropriately seems equally 'right'.
In the end, just as we may avoid accidents by trusting our fear, other successful use of logic rests upon developing a trustworthy relationship with other emotions, often this emotional sensation of harmony. In more modern terms, rationality depends upon finding ways to resolve "cognitive dissonance".
Cognitive dissonance experiments themselves seem to indicate that whatever behavior we exhibit that cannot be explained rationally to ourselves becomes part of our future reason, usually in the form of perceptual bias. If we work harder than expected and are not paid more we retroactively find the work more pleasant or more meaningful than if we were paid more (see Festinger). Boot camp and hazing create a sense of duty. Seemingly perversely, removing the threat of punishment makes people feel unsafe (see studies of early Naval discipline, of the aftermath of Stockholm syndrome, and of the domestically abused), because we internalize compliance as a value when we answer to authority in defiance of our own logic. Habit becomes logic the same way logic becomes habit.
This can be used as a basis for a general theory of learning via emotional adaptation, where we are evolved to incorporate reason from outside sources to exactly the degree we do not already have it. So when apparently effective external causes escape our ability to explain them, they become part of our understanding, instead.
It is tempting to point out the experience of using language to capture logic. We can try to identify reason with verbally-processed motivation. But we see at the same time how verbal processing can be used to whip someone up into a frenzy or impart dread. So this is not a useful contrast, either, just a trend.
Tradition has often tried to make a distinction between the chaotic emotions and the calmer reason. But really there are only a range of emotions from the more chaotic to the most calm, and the latter are more trainable by experience.
So we have no choice but to act from emotion, which has been trained by experience, and therefore incorporates reason. They are layers in a mediated system, with reason relying upon emotion for its acquisition and enforcement.
