Happiness is nice but at the end of the day it is just a feeling. We should focus on maximizing more objective things, people's physical and mental capacities in particular, as well as whether people are free to make choices for themselves as it's hardly meaningful if a person is only capable of doing good when others are ordering them about, it may even be worth them making some terrible decisions in the short-term if the practice may lead them to make good decisions autonomously in the future.
The key to a meaningful life, a good life is to be able to do things both things one wants to do (which does tie into happiness) and things that in a more objective sense make a person a better and more able person, this would include studying (not just for your career but for making yourself a better person), healthy eating, exercise, and being willing to be helpful to others when you can.
Happiness isn't bad or wrong. It's useful and is helpful to an extent. If you're happy while doing the right things that will make it easier to do them well. If you're miserable while studying you won't retain the information as well as if you feel positive feelings of fascination.
However, happiness isn't inherently good. A person abusing heroin may be very happy, but that doesn't mean they are doing good by injecting heroin (this is from an ethical or moral perspective, whether or not we should allow people to use heroin is for another subject). Aside from effects on others they are handicapping their own capabilities and so doing harm to themselves.
I will supplement MasturDbtor's argument above with Analects 16.5:
'There are three things men find enjoyment in which are advantageous, and three things they find enjoyment in which are injurious. To find enjoyment in the discriminating study of ceremonies and music; to find enjoyment in speaking of the goodness of others; to find enjoyment in having many worthy friends - these are advantageous. To find enjoyment in extravagant pleasures; to find enjoyment in idleness and sauntering; to find enjoyment in the pleasures of feasting - these are injurious.'
I support happiness, but only if happiness is for a good reason. If your country is in danger and you're happy, you're a horrible citizen. If you see suffering and you're happy, you're devoid of compassion. If your If your parents are ill and you're happy, you're unfilial and morally bankrupt. Unfortunately, those who seek happiness as an end goal can certainly gain it, even in such situations.
With the exception of hurtful behavior, we should strive for happiness. Nothing is worth anything if we're unhappy. To believe that we've got a greater purpose than to keep ourselves and the people around us happy is to amplify our importance in the grand scheme of things. We have short lives. Why not be happy?
As for happy heroin addicts, I also believe that everything should be legal. Adults should be allowed to make their own informed decisions. If that's the way a person chooses to live their lives, who is another to interfere? It's a personal choice everyone should be given the freedom to make.
As for utilitarianism, it's not happiness, but greed, capitalism and materialism that have lead to its downfall.
When someone says that people should choose to live a meaningful life rather than pursue things that make them happy they are ignoring the fact that making the choice to live a meaningful life is in itself a pursuit of happiness.
The actions we undertake either bring us pleasure or ease pain. If you are brave enough to strip away all the superfluous descriptions surrounding why someone did something you will find at the core there was a pleasure gained or pain avoided.
We are controlled by stimuli internally and externally from deep within our primitive brains, from our higher intellect, the environment around us and some would say, from the eternal energy that powers the field that defines us.
In any case we are driven by these stimuli and have gradually learned that putting off some of the more primitive pleasures for a while often leads to a more lasting deeper pleasure that exceeds the utility of some of the more primitive pleasures.
Read more at Pleasureworks: Pleasure is the Highest Good