The sun is, in any culture, one of the highest regarded symbols of humanity.
In any mythology, you will find that the sun is depicted as one of the highest in a pantheon of gods. The sun gives light, heat and life. Ancient civilizations revered it as a symbol of fertility, vitality, and every other quality that gives way to life. The sun symbol can be seen everywhere nowadays. Knowingly or unknowingly, people have incorporated the revered sun-design on many common objects we see today.
Sun tattoos are sought after because they take the round sun symbols and use it to depict so much more.