An emotion is a holistic physiological reaction to a perception.  It is the sum total of your autonomic, visceral, and hormonal reactions to something you perceive to have occurred.

I don’t say that emotions are only this, because it would be incorrect to describe something as “mere” which is not mere.  Your emotions are not merely a physical phenomenon because there’s nothing mere about physical phenomena.  They have meaning.  But just because an emotion has meaning does not mean it is more than it is: a reaction.

As such, emotion can never justify action.  You can never reasonably do something because you felt a certain way.  I don’t mean that you should not, I mean that such a statement is meaningless.  Because your emotion is only a reaction to a perceived stimulus, it is wholly a function of your perception.  If you perceive x, your emotion (e) is f(x).  e = f(x).  Your emotional function may be complex, but it has only one input and it is a function of only one variable.  To say you acted a certain way because of your emotion is to say that your action (a) was a function of your emotion.  g(e).  But if your emotion was a function of your perception, then a = g(e).  If a = g(e), and e = f(x), then a = g(f(x), at best.  In this case, you’re at best saying your reaction was an inevitable product of your emotion which was an inevitable product of your perception, so citing the emotion is a cop-out.  Your reaction was a function of your perception, and the emotion was just a formulaic interlocution.  More likely, though, you can’t even claim that.  Your reaction was produced by a combination of your emotion and other factors (a = g(e, c)), in which formula your emotional reaction is still an inevitable formulaic interlocution.

A simpler way to say this is that the emotion is just the convoluted path by which you get from perceived stimulus to reaction.  To say you acted because of your emotion is equivalent to saying that your arrival at the restaurant was due to the road, rather than due to your perception that you were hungry.

So, to speak honestly, you did not punch that man because you were angry at him.  You punched him because you perceived him to have slighted, injured, or threatened you.

What did the emotion contribute, then?  If an emotion is solely a function of the perception which triggers it and adds no information to the system, what does it provide?  We might say it provides amplification.  Emotion is a booster, an additive function.  You have a well of potential energy inside you (metaphorically speaking).  When a stimulus comes in, it initiates a function which produces a reaction, and the reaction would carry only the energy of the stimulus, except your emotions add to the energy of the stimulus energy from your own internal store, to produce an output stronger than the input.  (Helicopter pilots, you are familiar with hydraulic boost servos commanded by shuttle valves.  Think along those lines.)

We can also say that emotion adds information to the event, specifically a flavor, a visceral quality.  It adds a rhythm track and chords to a melody.  It adds umami and body and mouthfeel to a dish that otherwise might be just a bouquet of spices.  Note that this is not new information, though.  The emotion, with all of its subtlety, was already in you, and you added it to the experience to boost the experience, but you’re giving yourself something you already own, from your own internal supply, to enrich your experience.

This is all very fancy, but the upshot of it is that an emotion, being only a reaction to a perceived stimulus, is only as valid as your perception of the stimulus.  If you are angry that this man slighted you, well, you probably shouldn’t be, because he probably did not slight you.  You only think he did, because you are incorrectly perceiving the situation.  If you indulge your anger under those conditions, your anger will enhance your reaction to the perceived slight, and you will get all worked up and start lashing out in response to something that never happened.  That makes you insane           .

Likewise, if you become attracted to a man who seems to be perfect, and you allow your emotions of infatuation to empower your reaction to that attraction, you will dive headlong into a relationship.  But if your perception was in error—if he was in fact not perfect, but quite a poor choice—then your emotions have helped you react to something that never was.  You’ll tumble head over heels for a man who never existed.  That makes you insane.

Emotions have meaning, and they have value, but put them in their place, by recognizing what they are.  Books are probably more valuable than a bookshelf, but you don’t stand your bookshelf on your books.  Use reason to establish that what you perceive to have happened actually happened, and use reason to determine what your reaction to that should be, then use emotion to empower that (correct) reaction to a (correct) perception.