Some people feel that they don't deserve to be happy. Sometimes people may say, "It's so selfish that you don't care about your flaw!" or "I don't deserve to be happy because people in Africa are suffering!"

But,

"It's not selfish to seek happiness. IT IS SELFISH TO BE MISERABLE! Happy people are more thoughtful and more considerate. It's unhappy people who are preoccupied with themselves." - Andrew Matthews, author of at least 5 bestselling books about happiness.

To make other people happy, you have to first be happy yourself, and happiness can only be achieved through positive thinking. In other words, if you want to be a happy person, you have to be optimistic.

But why should I be optimistic?

1. Optimism and pessimism are energies that spread. When you are happy, this energy will spread to the people around you. My own opinion.
2. As mentioned in the above quote, miserable people tend to be more preoccupied with themselves. I know this statement is true because i reflected on myself. (I included this sentence on purpose for those people who judge me and say things like "you never reflect on yourself") Optimism leads to happiness.
3. We live to be happy (and perhaps make the people around us happy), so why not be optimistic?
   

However, some people feel that optimism is plain stupid because it's like deluding ourselves.

Optimism is definitely not self-delusion. Self-delusion is self-delusion. If everyone says that your attitude sucks and you tell yourself "My attitude is awesome and everyone loves me", then you're not being optimistic, you're plainly lying to yourself. If this really makes you so happy, go on.

Optimism is when everyone says that your attitude sucks, and you tell yourself, "Nevermind, it's a good learning experience. Everyone's input allows me to change for the better!" Difficult, but possible.

Some people feel that being optimistic is impossible, because (insert an excuse).

It's natural that we tend to focus on reasons why we cannot be optimistic, and then we say "that's why we can't be optimistic!" But, if you're driving a car and there's a split in the road ahead, focusing on the direction you don't want to go will bring you to the direction that you don't want to go! Instead, focus on why it is possible to be optimistic. Sometimes it's really difficult. But it is possible. If not, then just don't think. No thought is better than negative thoughts.

Some people don't want to be optimistic because if they are to be optimistic, they have to change their natural thought processes. In this case, force yourself! :D

One last thing: The people who cause the most unhappiness are pessimistic people. It's not selfish to be happy.