Mental illness often results from excess emotion. The overflow of emotion doesn't just drive mood disorders like Major Depression but fuels most psychological problems: phobias, anxiety, trauma, hoarding, obsessiveness, borderline personality disorder, and drug and alcohol abuse.
Why (oh why) were we made to feel so much? It's as if our wiring (which, when given the options of emotion and reason, gravitates to the former), were out to screw us.
The popular answer is the evolutionary one--that emotions have helped us survive. When we lived in the wild--with monkeys and mastodons and tigers--we needed emotions in order to react quickly to dangerous stimuli. If faced with a tiger, it's better to be rocked with a fear so strong it triggers a rush of blood than to sit around and theorize about the threat. We developed an emotional system because it could induce quick responses to danger (for theorists on emotion and evolution, see Antonio Damasio, Joseph LeDoux, and Robert Trivers).
But the claim that emotions keep things alive is too simple. After all, you can name a lot of efficient, enduring response systems that don't include emotion. Rivers are one--they skirt serious barriers and survive through history. Or consider an ivy plant. It has a very good sensory system and no love, fear, or drama to weigh it down. It winds itself up from the ground, over rocks, through the locks of gates, finding places to cling. It can endure weather changes, feed itself, and grow new cells. That's a solid response system not driven by emotion. So, again, why were we built to carry the burden of so much sentiment?
An evolutionary answer with a bit more detail is that we're animals: more aggressive and self-conscious than rivers and plants are. Aggression and the desire to survive that comes with selfhood helped scoot animals up the food chain. If you want to create a system that works hard to survive, make it consciousness and emotional. It will want to keep itself around.
On top of that, human beings are the most self-conscious animals. This makes us increasingly invested and crafty in our need for survival. We developed basic emotions (fear, joy) like the other animals. But then we developed a more complex rational system too, in which we could imagine our own past and future selves. It was the ability to reason about old and future selves (to set traps, and not just run from tigers) that allowed us to dominate the food chain. Rational thought helped us shape the world for our future: rerouting rivers, breeding plants, caging tigers.
We now have two highly developed systems: reason and the emotional core still sitting there, like the primitive animal inside us. As said above, mental illnesses often result from an imbalance in those two. And emotions kick us in the ass these days for a number of reasons. For one, even though emotions like fear used to be helpful in the wild, they're less efficient helpmates in modern civilized life. Of course we still have plenty to fear, but our threats are not usually immediate, like a tiger, but rather distant, like money and war and homelessness. The old fight-or-flight system is inadequate to the modern threats. You can fight a tiger; but you have to work hard, for a long time, to fight a financial crisis or the threat of terrorism. So, a lot of us suffer from a more generalized fear, or anxiety. There's a sense of danger without a practical opportunity to respond to it quickly. Perhaps emotions get out of whack today because they bubble without an effective outlet.
In turn, therapy these days often focuses on keeping our systems in balance--balancing our emotion and reason. One nice concept along this line comes from Dialectical Behavioral Therapy (DBT). DBT has established the concept of the "wise mind," which is a useful integration of the emotional and rational selves. We get to "wise mind" when we're able to step back from emotion and reason with it. The wise mind puts emotion and reason in conversation, or uses reason to calm emotion down.
We likely have emotions because they help us survive. But they also tend to drive us crazy when given too much reign. Of course what I'm saying barely scratches the surface. After all, the other reason why we developed emotion is that emotion helps build relationships and bind communities. We would not be able to coordinate our goals so well if we did not love and fear and trust and feel a sense of pride. In this light, here's post #2, which focuses on the social use of emotions.