If happiness is so wonderful to experience, it'd be natural to completely shun away from its polar opposite, sorrow. Depression itself can be classified as a mental illness. People prefer happy endings over sad ones. Nobody comes up with a book titled "How to be sadder in X simple steps!" and sells millions of copies... or at least I don't know of such an instance yet. The proportion of people that like to indulge in feelings of sadness or anger are seen as "abnormal"; look how often the emo/goth kids get teased!
Or are we also are foolish for choosing to not even look sadness in the eye? Why are we so afraid to be sad or angry? Most of us have been conditioned to "look on the bright side of life" and "smile even when it hurts". Is this correct behaviour?
In some cultures we've also been taught to seek out balance. To strike balances between work and play, self-indulgence and altruism, conformism and rule-bending... so why not a healthy mix of joy and sorrow? It's certainly wonderful to feel happy, but is it desirable to feel that happy in the long run? It sure is a nice feeling to see the beauty of the world around us, but we sometimes miss the weeds hidden in the flower garden that if left untreated could overwhelm the whole place and leave no room for the pretty sunflowers, roses, tulips et cetera. Indulging in optimism can sometimes make us neglect pressing problems, that if discovered too late, could cause us to crash and break down.
This is what most of us like to refer to as procrastination.
In the less likely cases when you are really nearing a nervous breakdown with work still left undone, it could be more beneficial to put the work away temporarily and give yourself a quick break. Get yourself a little period of entertainment, with restraint of course. Then you can return to your work as a more productive person in a lighter mood, and are able to tackle the work much quicker. However, if you decide to procrastinate in a situation where it'd be much faster to get the problem over with, it'd be more likely that the problem would worsen. It does feel good to not be dealing with problems and just be enjoying yourself, but the weeds would have more time to grow and spread. Usually it's better to just get your hands dirty and uproot the weeds quickly.
Moving on to another point: sometimes we think we are happy by smiling a lot and saying nice positive things and living healthily, when in fact it's merely nothing more than a cover-up. Just like putting concealer over your face to cover your dark marks and scars, you may appear to have perfect complexion, but in fact the flaws still exist, and might even worsen with too much concealer. Indulging in "happy" activities might help you alleviate your mood to some extent, but the root cause still persists. It's usually more effective to acknowledge that something's not right and vent out your frustration, rather than bottle it up and hide it with fake smiles. It'd be much worse when you eventually just cannot hold all the negative feeling in, and just crash. Expressing your sorrow works better than trying to dilute it with sickening amounts of joy.
We'd usually prefer a utopia where very few problems exist and we're mostly happy, but we know just how unrealistic it is in this world where problems pop up rapidly. And to only "look on the bright side of life" can distort our view of the real world. It's hard to deal with real world problems if you aren't even able to see them through your rose-tinted glasses, right? Being too indulgent in joy can make you miss the plight of others: social inequality, poverty/disease-stricken populations, jobless people with families to support... the people who selflessly devote themselves to such causes do not only pursue self-satisfaction, but also sacrifice some things for the sake of alleviating the situation of others. The "downgrading" of an individual's happiness in exchange for the greater improvement of many other's situations is a good deal, especially from the utilitarian viewpoint!
And who says that you can only see the beauty of everything when you are happy and never sad? Without first experiencing sadness, how would you appreciate happiness? Sorrow provides a standard to compare against. If you have never felt like crap before, it'd be harder to understand just how lucky you are to be enjoying your improved state. Only when you remember how low you were would you better understand how much higher you are now.
In short, sadness and happiness are not mortal enemies, where one must be forgone completely in order to embrace the other. Instead one should not be attained without the other. What we should strive for is not total happiness, but rather a healthy mix of positivity and negativity, allowing us to keep ourselves grounded to reality while maintaining a healthy outlook on life. To drown in sorrow is to completely give up on life and what it can offer, but to indulge in excess happiness is to lead a warped version of life.
A good cup of coffee has to have the right mix of sweetness and bitterness; too much of either renders it undrinkable.