I think that a lot of people have a general misconception about the make-up of the populace. Many people want everybody to be happy and smiling. They think that this would lead to a better world. Would it? Is this what the individuals want? is this how the world should be? Is it a therapist's job to help? is this ethical?
Simply put, I think that every person has the right, ethically, to be happy or unhappy. It's interesting, because even the word happy is subjective. Sometimes being angry is very positive. Better said: Someone can be happily angry! They need to expel/communicate/show that anger. Some people revel in being depressed, as if this is their main mode of expression. And that's OK! As long as they aren't impinging on someone else's happiness, then they can behave as they see fit.
The world would be much more boring if everyone were stereo-typically happy. Sure, there wouldn't be war, but the lack of diversity in emotion would lead to a lack of diversity in thought and behavior. It almost follows that this homogeneity of affect could depress people. So, to answer the first posed question - no, I don't think that the world would be a better place.
To some degree I think I've already answered the second question. It's true that one would not know what happiness was without sadness, anger, or melancholy. So, in effect, happiness would cease to exist. Back to the question: It's hard for me to believe that a lack of diversity of emotion would be valued. This would mean that everything in everyone's life goes well (which is impossible) or that they are a master of their own personal emotional status.
I think that it is a therapist's job to help people learn to control themselves, to keep their "stuff" within arm's reach, not letting get it all get out and affect others. This could translate into greater societal change many years down the road. I think that it's also important to mention that most people can do this to some degree. They have the ability to self-regulate. True, things can get away from them and touch their families (who can also be within arm's reach), but hopefully those relationships are compassionate and forgiving enough to deal with it.
I do not think that it is ethical to try to change clients. It's just not our job. Our job is to help the client understand themselves enough to allow the change to occur, if wanted. This type of treatment is deep therapy at its purest. No single technique or type of psychotherapy can achieve this. Fortunately or unfortunately, this type of change normally takes time and a good set of tools. Clients must choose to change themselves. We cannot force it. There is no mold that we should be pushing them into. I think that that is a main problem in current strategies in psychotherapy.
I made a point in a previous paragraph that requires a sentence or two of elaboration. Everyone being happy would lead, most likely, to depression. If this is in fact true, then a world that is only happy is almost, by definition, unsustainable. It's impossible for people to be happy forever or to be happy constantly.