"Om namo bhagavate vasudevayah"
 
The present life of a living being is just one among many. A soul passes through many bodies to take one after other births. We also know(the post has been discussed earlier) that every soul is part of the Supreme Soul, the Supreme Purusha, Bhagavan Shri Krishna. Here one question may arise, a living If Purusha (Atma) which has been started from the Supreme Purusha and which is destined to meet the Supreme Purusha, then why it is getting lost in the cycle of births and deaths? 

A soul got attached to the body and nature around the body. It enjoys all the materials and considers this material world as his place and so he comes into this world again, he is reborn.
 
The same happens with the soul. It lives in the body and comes into contact with three Gunas and then it forgets its origin.
This is has been very clearly mentioned by Lord Krishna. Please have a look at the shloka.

Ch13.Sh22
purusah prakrti-stho hi bhunkte prakrti-jan gunan
karanam guna-sango 'sya sad-asad-yoni-janmasu
 
Living in material nature, the Purusha (individual conscious) enjoys the modes born of Prakriti(matter). His connection with the material nature becomes the cause of birth and he meets good and evil.
 
The shloka mentions the following facts
1. The Purusha (individual conscious) enjoys the modes born of Prakriti(matter).
2. Purusha gets connected with the material nature(Three Gunas)

The living being enjoys this material world, developing hundreds of desires, Some of those desires fulfilled many most of them not. To fulfill those desires the soul has to acquire another physical body. Apart from the attachment to the material world however there is another reason for a living

A soul got attached to the body and nature around the body. It enjoys all the materials and considers this material world as his place and so he comes into this world again, he is reborn.
 
A human does his karma with the desire of wealth, house, car, etc. Some of those desires are fulfilled, but most remain unfulfilled. Living being hence has to acquire another body to further fulfill those desires which it could not do earlier. That is another matter that when in another body the living being piles up with even more desires and so he has to born again and again. 
 
But since Gunas are of different types. Human beings contain different proportions of these Gunas in their body. For this reason, they possess different kinds of desires.
  
There are three Gunas:
1. Sattva (mode of goodness)
2. Rajasa (Mode of passion)
3. Tamasa ( Mode of Ignorance)
 
Based upon how one soul attaches itself with three Gunas decide which body he is going to born. It can be summarized in the following way.
 
1. Attachment with Sattva Guna(mode of goodness):
One who attaches with Sattva Guna(Mode of goodness), takes birth either in Higher planets like, Deva Loka(Heaven), Pitriloka(Places of ancestors) or he takes birth as an elevated or highly placed human.

2.Attachment with Rajasa Guna(Mode of passion):
 
One who gets attached with Rajasa Guna(mode of passion), takes birth as a human in the next life. Again what kind of Human body he is going to get, depends upon his karma.
 
3.Attachment with Tamasa Guna (Mode of Ignorance)
One who gets attached with Tamasa Guna(Mode of Ignorance) takes birth as a lower human or other species.
 
Till the time this soul is attached with this material world into three Gunas(qualities) is keep taking birth. When this soul realizes that this world is not his natural position and then bring himself where he has not left with desires and hence loses the purpose to return back to this mortal world, he gets free.

There is another reason causing rebirth and that is the negative actions done by a human. By doing so he incurs liability as a physical being and has to take one or more births to pay the consequences of his past actions. But then this is not part of this subject but the subjects of "Karma" and would be discussed separately.
 
"Shri Hari Om Tat Sat"