Emotion and rational
thinking are, to a certain extent, mutually exclusive.
Clouding the
waters
When we get emotional
about something, our ability to make rational decisions (which we will
look back on and agree are good decisions) has a strong tendency to fail.
Consider the things
we say. A person who is 'hot-headed' is considered to be rather emotional
and likely to make rash decisions, whilst a person with a 'cool head' makes thoughtful,
considered decisions. We talk about mental processes being 'clouded' by emotional states.
Emotion is a chemical
state in our brains which we experience as basic 'feelings'. Those same
chemicals inhibit our higher cognitive capabilities and limiting what we call rational thought.
More emotion,
less judgement
The effect increases
as emotional arousal increases. Think of the ardent lovers who have
married in haste, only to repent at leisure (Las Vegas seems to do this to people more than
most people -- but then Las Vegas is designed to stir the emotions).
The same is true
when a person is frightened, and the Fight-or-Flight reaction gives them the
ability to fight for their lives. Many homicides are committed whilst the perpetrator is an
extremely angry state. Temporary insanity is a not uncommon plea, as the extreme emotion
literally makes the person unable to make any socially acceptable decisions.
So what?
If you want someone
to make a rational decision and they are in an emotional state, then help
them calm down (but do not just say 'calm down', which is a rational appeal). Speak to the
emotions with emotional words and then slowly become less emotional. If you have time, the
simplest approach is just to wait. Emotions do not last and the person will eventually cool
down.
If you want the
other person not to think about the decision, then get them into an emotional
state. A sales person will get their customers excited, hopeful or into a state of lust.
Politicians
will invoke fear or the desire to help their fellow humans. 
