It's obviously a lot easier to become happy if you know what it is that makes you happy. A first simple step that can help you determine the components you need to have in your days and weeks to be happy is to determine whether you're an introvert or an extrovert. For some people the answer to this will be very apparent, but think about it again because there is such a thing as a shy extrovert and many introverts are very confident and skilled in their engagment with others. The best way that I have come across to determine which of these categories you fit into is to ask yourself when you feel your best and most energized. Is this after a day spent with a full house of friends or is it after you've had a quiet walk in the park?
This is not to say that as an introvert you do not enjoy spending time with other people, it's just more tiring for you than for an extrovert. An extrovert on the other hand, will often feel more washed out after a day spent alone.
Both introverts and extroverts need time in other people's company as well as time alone for reflection, but the balance can be different. Most importantly an absence of time alone can make introverts feel very unbalanced and exhausted, whereas spending even just one day entirely alone can make an extrovert feel depressed for no apparent reason.
Sometimes it's simply a mistake in getting this balance right that makes us unhappy, but if you are not aware what it is that you need it is harder to improve. If you are an extrovert, you would do better to work in busy environments with lots of people around, and usually you are not overly bothered by noise. As an intravert, you tend to thrive in quiter environments and can do better when left to your own devices. No matter what your job though there are things that you can do to make it fit better with your extrovert nature.
Extroverts will find working from home, on their own, a challenge, but there are ways to overcome this. You could spend some of your working day in a cafe, or arrange to meet up for lunch with a friend, or go for an exercise class where you could befriend and chat to some of your class mates. Either way make sure that you incorporate some social activities in your day.
Similarly an introvert might find a job that involves a lot of talking and wining and dining of clients difficult. In this case I would recommend having some quiet time when you get home or during your lunchbreak - go for a walk or for a run, or just read your favourite newspaper for a while, until you recuperate.
Once you become aware of the correct balance of activities, feeling happy will be easier and you will certainly have fewer down days.