> Emotions are, in fact, rational, inasmuch as they act in the person's best interest.

Nope. Quite the contrary in fact. Emotions are, in fact, emotional, inasmuch as they act in best interest of one's genes. Such as raping multiple women and getting away with it. Or killing another's tribe's men and women and children to acquire their resources. In modern times the same game is often played out although not to the same extent.

> Take, for instance, fear. If you are face to face with something dangerous, you feel fear, which drives you to get away from the feared object--a rational response to danger.

First of all, fear is not a result of facing with "something dangerous" (which danger was appraised at the cognitive level) as the emotion of fear is instantly -- with no conscious forethought, much less "rational response" -- triggered in response to sensory cues that are indicative of danger in an evolutionary sense. A fast moving snake for instance. In modern times however the 'cues' for fear often have underpinnings on one's socialized identity (such as fear of getting rejected by whomever).

> Anger works on a similar principle;

As explained above (and below), no emotion works on a rational principle.

> when you are slighted or hurt, you feel anger, which drives you to attack the thing that hurt you. Again, rational response, assuming no external factors (i.e., the person who hurt you is your boss and hitting them back doesn't end well).

The emotion of anger itself -- which viscerally drives you to kill, maim or wound a fellow human being come what may -- is an emotional response sourced in the survival of the individual/species. It is not a rational response "in the person's best interest". Indeed, your tactically placed nebulous qualifier "assuming no external factors" tacitly implies that such an (instant) emotional response needs a (slow) cognitive rational appraisal to "control" it, lest one end's up killing, maiming or wounding the other person leading to grave consequences (such as getting hurt back, or being sent to jail or losing what one holds dear).

> All emotions behave like this.

As explained above (and below), no emotion is (much less "behave like") a rational response.

> They take certain inputs and return certain outputs.

The inputs are sensory cues (and also thoughts/beliefs themselves), and the outputs are war, rape, murder, domestic violence, etc. as demonstrated by human history.

> They are inherently logical, and assuming that the inputs are known and complete (you're not missing any details, in other words) they result in rational decisions.

As the original comment is about "source of" emotions like fear, you are going on a tangent here talking about "result[ing] in" rational decisions.

> The reason we think of emotion as irrational is because Spock said it, and Spock only said it because he correlates emotion with the Romulans and their insanity.

Not everyone in this world is familiar with American Television, and yet all throughout the world peoples consider emotions are irrational visceral response (hence the need to cognitively "control" it, albeit with feeble success)

> As a final note, "Rational" really just means picking the option most likely to achieve your goals. By this measure, joy and sadness are both rational, as one indicates to others that you enjoy something (hopefully leading them to do it more often) and sadness provokes the opposite reaction.

You are watering down the word rational. Going by your definition if say a sociopath has set his "goal" to rape as many women as he can, then picking the best option in every day life situations to achieve that goal would be considered "rational". Just so there is no confusion, here is what the word means:

1 based on or in accordance with reason or logic: I'm sure there's a perfectly rational explanation.
• (of a person) able to think clearly, sensibly, and logically: Andrea's upset—she's not being very rational.
• endowed with the capacity to reason: man is a rational being.

Also sadness is a silly reaction, and has no rational use towards any felicitous goals.

> Why do you think we're hardwired to get upset when we hear crying? It helps the species as a whole to achieve their goals if we work together to avoid things we don't like.

First of all as the "woman with no fear" has plentifully demonstrated, emotions are not "hardwired". Secondly, not everyone gets upset when hearing crying in any situation. Lastly, as a sensible goal of the human species can be said to be live in peace and harmony, I fail to see how the aforementioned emotions can help achieve the said goal (to the contrary, they lead to war, rape, murder, sorrow, and the like).