Human is one of the species from 84 Lakh (8.4 million) in this universe. As we are born on this earth, it is an indication that we will die. This is God's law that whatever is born, it will die. Look around yourself, is there anyone who was born but did not die?
While we are alive on this earth, all of our deeds through physical action, mental thoughts, and speach are contantly being recorded by Chitargupt (Lord assigned by God to record actions of human on earth). When we die our karams go with us; we are presented in the court of Dharamraja (the Lord of justice), in the court of justice our recorded mental and physical actions are read by Chitargupt and then Dharamraja decides our punishment or reward. Some go to higher planes while some go to lower planes.
In short, if we did not do good deeds on earth and have not remembered God during our time on earth, then even before presenting us to Dharamraja we are beaten by messengers of death when we die. They beat us severely but we cannot escape. Then we are presented in the court of Dharamraja where he instructs his men to put us in various kinds of Hell (where extremely severe punishments are given, like burn in fire etc.) before we will be pushed into another cycle of 84 Lakh species and then finally a human birth with lot of economic, social, and physical problems.
If we remembered God during our earth time, then we will be given a position in Heaven before we are given another Human birth on this earth in order to further progress our Spiritual journey. If we did many many good deeds we might be shifted to other higher spiritual realms (above Heaven) untill we reach the last 14th realm which belongs to God.
Even human births are of different kinds, some are begger but some are kings, it is because of their previous birth's deeds. But whatever are the circumstances in human birth, we have one most important power which no other species has and it is Free-Will. Free-Will could only be utilized in human birth using which we can change our destiny.
The main reason for this human birth is ONLY to do meditation of God in order to get ourself free from the bondage of karma. As per our previous karma we would get pain/happiness, rich/poor etc, but keep in mind these are only the rewards or punishments for our previous deeds and always remember that we do have free-will even though it would be limited because the circumstances around it are the results of previous deeds. It is ONLY in human birth that free-will could be utilized and we can do worship of God. If we do not do that, then this human birth will go in vain.
Death will come anytime, no matter how good food you take, how good protected house you have, how good physically you are, how good doctors you have. In other words, no matter what, death will come anytime. If death is sure to come, what is the use of so many bad deeds that we did just to collect money, property, relatives. Nothing is going with us. Whenever we die, only our karma go with us. Therefore, it is must to remember HIM (God) before we leave this physical body.
There are people who do not believe in the above. They do not believe in Chitargupt, Dharamraja, Karma, 84 Lakh species. But when we will die we will came to know that these things do exist and we have to repay for our karma/deeds but then it would be too late.
e.g[ Once there was poor gardener and he found a bag full of beautiful stones. He used all the stones to fly away the birds from the fruits. During this time, her wife came to him and saw the stones. She said the stones are beautiful and requested her husband to allow her to take the last stone to home to keep it. Gardener granted the approval as only one stone was left with him. His wife bring that stone to her house and was starring the stone while sitting in the house's door. Then one of the jeweler saw the stone and questioned the gardener's wife. The jeweler offered thousand dollars for that one stone. The gardener told his husband and now, both of them realized the stones that the gardener used to fly away birds were expensive. Now, both started to feel very bad because the gardener used so expensive stones for nothing but apart from being sad he could not do anything. Same way, suppose the stones are our breaths. We do not understand the importance of human-birth and its limited breaths, so we're just wasting the expensive breaths on nothing. If we were aware of the importance of this human-birth and breaths, then we would have utilized them in positive manner by remembering God. ]
e.g[ Pigeon closes his eyes to shut out the sight of the cat when the cat comes to eat him. Pigeon thinks that cat cannot see him because he cannot see cat. Of course, the cat remains, and the poor pigeon realizes it only when the cat takes hold of him. Same way, denying the fact of Death, Karma, Court of Justice does not mean that they does not exist. We will came to know when we will die but then it would be too late. ]
e.g[ A king wanted to choose best responsible person for his thrown, so he held a meeting with his 3 sons. He ordered his sons to undergo the journey of all religious places within one year and he also gave them 3 horses (one to each) which they had to use for travelling. First son did not start his journey and just fed his horse extremely well and converted his horse into very fat. Second son started his journey and he tried to complete the journey as soon as possible and therefore ignored the health of his horse. He over-use the capacity of his horse and eventually his horse died before completing his journey. Third son was very wise, he calculated that if he travel this much distance each day he would be able to complete his journey on time. This way he travelled pre-planned distance each day and also took care of his horse by giving him proper food and rest. Eventually he completed his journey within the provided time.
The third son who completed his journey on time by visting all religious places was chosen as next King. The one who did not start his journey was given punishment by the king. Same way, God has given us this body (as horse) so that we can complete our spiritual journey to God within this human birth time. If we did not remember God during our time on earth then we are like the first son and therefore we will be given punishment by Lord of Justice by either throwing us in Hells or giving us birth in lower species. If we act like second son and due to Virag we did hard penance of God without taking care of our body, then we might get another chance (another human birth) by God so that we can continue our journey. We are supposed to act like third son by remembering God and taking care of our body by fulfilling his needs. i.e When we were in mother's womb, everyone (regardless of caste, creed, and religion) promised God that we will do God's worship for at-least 2.5 hours out of 24 hours. This is our main duty that we have to do along with our wordly duties. ]