We all get into relationship. We at times move out of them when the needs (emotional,physical,mental,monetary,etc.) are not met. We keep searching for that one life partner. Why do we(human beings) need that one "Life partner"?
The root cause is almost certainly procreation. Our relationship tendencies developed along with our biology. Humans tend to have one child at a time, and human children mature very slowly. This suggests that during most of human history, it would be advantageous for a female to keep a male with her, helping to provide food and protection for herself and her children. The strategy for the male is either to play along, or to breed so quickly as to outweigh the devastating effect of a long maturation without sufficient protection. Others species, many of which breed in greater numbers, have different mating habits.