Why is human life so painful? Is it because of Kali Yuga or because of our Karma?

When a child is ill, the mother gives medicines to it. The child initially refuses to take the medicine owing to its bitterness. For a moment, the mother diverts the attention of the child through some play. Then again, the mother tries to feed the medicine. The mother will continue to do so, without giving up, until the child takes the medicine completely, wouldn’t she?
In the same way, God gives us sufferings to realize that the state that you live in is not your true state. When the sufferings become unbearable, he distracts you by giving you a sigh of relief. However it continues to trouble you and will not leave you until you realize your true nature which is the Self.
The simplest way towards this goal is by chanting the Mahamantra.