Except for some extreme situations(God forbid that) we can remain happy if we really intent to be so.
The best way would be to adopt the balanced situation. Both the extremes are to be avoided such as depression due to failure or over excitement for the success. Though it may appear difficult initially but with a little practice balance of mood is achievable.
"Life is easier when you enjoy what you do"
Tension, depression, sadness, jealousy, grudge, hatred and all ill feelings are enemies of happiness. Never invite them to befriend with you. All these ill things destroy happiness. Real happiness is got when you have peace of mind. We should embrace positive thinking and stay away from negative thinking. They play a big role in our lives.
I agree with the members suggestions on positive thinking which keeps oneself away from an unhappy mind. It is also possible to recall your days when you were at the peak of enjoyment. Be happy always.
Lead the leader
I have to have a sweet dish in my lunch but I couldn't. So you will start thinking about why I couldn't have sweets at lunch. Where did a do a mistake and how can I get it? Your thoughts will be around that only. If you have no wish to eat sweets. You had your lunch and you are not feeling hungry and no ill feelings. You are happy.
Try to be happy with whatever you have and try to see the people who are suffering more than you and thank God for all whatever HE has given to you and thank him for the same. That will give you happiness.