If the primary purpose of each of us is to pass on one's genes (i.e. to have kids), then many people today, including yours truly, are abdicating one's most important responsibility.
Among the people I know (friends, colleagues, and relatives), most of the people whose age is similar to mine have yet to get themselves involved in a relationship, let alone getting married and having kids. Judging by the amount of time they spend at work, this is hardly a surprise — not being able to leave one's workplace until long after dark and having to work on some weekends leave one little time for meeting potential partners and fathers/mothers of one's future child(ren).
Even when two people choose to get married or co-habit, they may choose not to have children. While some reach their decision due to their unwillingness to change their lifestyle, many choose not to raise a child because of the lack of time and money.
The problem with time and money is that one often has to sacrifice one to gain the other — to earn the amount of money necessary to raise a child, it is highly probable that both parents would have to work and to devote less time to raise the child; if one parent decides to stay at home to raise the child, the money earned by the one who works often is not be enough to cover the family's expenses. It sure doesn't help matters when the notion that half a million (US) dollars is required to raise one child is engrained in the minds of many people in town.
While competition drives the world forward, one gets the feeling that competitions begin way too early in a child's life today — people fight over spots in the best pre-schools when some of those being enrolled are yet to be born, children as young as five being drilled on how to perform "well" in admission interviews for places in primary schools (I took part in one of these interviews when I was five without any prior training, if I remember correctly). The unwillingness to subject a child to such intense competition at an early age may be another reason why many people are reluctant to have kids. (The local education policy, and the almost-yearly changes to it, may be a stronger deterrent. A stand-alone article may be needed to explain why.)
Similarly, parents of different children often compare their children's capabilities and achievements. The fear of having to subject themselves to such competition can also deter some people, especially those who have an inferiority complex, from having children.
From time to time, children engaging in unruly behaviour. Some may feel that they may not be capable of instilling discipline into their children and teaching them morals and the ability to distinguish right from wrong. The task of keeping one's children to behave alone can pose an enormous problem for many — when a child behaves badly, the parents' severe punishments can drive a child to become more rebellious or be fearful of everything; on the other hand, if parents do not dole out any punishments when a child behaves badly, the thinking that one can get away from anything will develop in the child's mind, making the task of more difficult in the future. This way of thinking can sway some into not having children.
The above are some of the reasons that drive me to have no plans of having children. While there is a (very slim) chance that an article describing the joys of raising a child will be posted n years from now, don't count on that article appearing any time in the near future.