My mom told me it was hard to make me smile since I was born, why some children are bron like that?

Personality types. A child's temperament is often noticeable shortly after birth. There are very relaxed, laid back babies and very high strung, irritable babies. Personality types such as being serious is probably at least partly inborn.
Make it happen. Smiling sometimes starts as a conscious effort, but will grow to be a lifestyle if you continue. Smile at yourself in the mirror whenever you have a chance.
A Hidden Gift? Things that don't come naturally may be the areas in which you have your greatest potential for growth. If you work on smiling more, you may find yourself feeling happier. You will probably find yourself spreading happiness to others, as well. Indeed, you may find that this become your greatest source of fulfillment. Maybe this natural tendency is actually a gift...