Just as there are four seasons, there are four ages in which we keep circulating, one after the other:
Satya Yuga: The age of truth. Only one religion (Sanatan Dharma) exists in the whole world. The Yuga lasts 1.728 million years and the lifespan of humans is 100,000 years. People are mostly selfsatisfied, merciful, and friendly to all living beings. They derive pleasure from within, see all things equally, and always endeavor for spiritual advancement.
Treta Yuga: The introduction of ignorance takes place in this age, but still only one religion exists in the whole world. The Yuga lasts 1.296 million years and the lifespan of humans is 10,000 years. People are devoted to rituals and severe austerities, but the principles of religion are reduced by 25%.
Dvapara Yuga: Decline in the truth and religious values take effect in this age, but still only one religion is followed in the whole World. The Yuga lasts 864,000 years and the lifespan of humans is 1000 years. People devote themselves to the study of the Vedas, are very opulent and enjoy life with large families, but the principles of religion are reduced by 50%. Dissatisfaction, untruth, violence and enmity increase.
Kali Yuga: The age of darkness, ignorance, and degradation. Many man made religions and deviations blossom in the World. The Yuga lasts 432,000 years and the lifespan of humans is 100 years at the beginning and this gradually reduces to 12 years. People tend to be greedy, badly behaved, obsessed with material desires, and fight with one another without any reasons. Only 25% of the principles of religion remain and these gradually decrease by the increasing principles of irreligion (mainly untruth, violence, enmity, dissatisfaction, following unknown/unproven/unseen characters as God).
We are currently 5000 years into Kali Yuga. As each day passes, man is becoming more ignorant and degraded. The result of which is more suffering for mankind.