It is something we are told to do by God. “Be fruitful and multiply” (somewhere in early Genesis). Having children is one of the outcomes of being married. It is part of what being married is for. Therefore, Catholics believe that artificial forms of birth control are against the will of God. Catholics believe that married couples should be open to the possibility of having children. Artificial forms of birth control nullify the very possibility of being open to creating life. However, natural family planning still leaves open the possibility of bringing new life into the world. Another problem with artificial birth control is that, while you won’t hear this much from “mainstream society”, some of the forms of “birth control” actually cause abortions. For example, and IUD irritates the lining of the uterus so that the fertilized egg cannot implant in the wall. That’s abortion.
Here’s what the Catechism of the Catholic Church says about this topic:
A particular aspect of this responsibility concerns the regulation of procreation. For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but it is in conformity with the generosity appropriate to responsible parenthood. Moreover, they should conform their behavior to the objective criteria of morality:
When it is a question of harmonizing married love with the responsible transmission of life, the morality of the behavior does not depend on sincere intention and evaluation of motives alone; but it must be determined by objective criteria, criteria drawn from the nature of the person and his acts, criteria that respect the total meaning of mutual self-giving and human procreation in the context of true love; this is possible only if the virtue of married chastity is practice with sincerity of heart.
Periodic continence, that is, the methods of birth regulation based on self-observation and the use of infertile periods, is in conformity with the objective criteria of morality. These methods respect the bodies of the spouses, encourage tenderness between them, and favor the education of an authentic freedom. In contrast “every action which, whether in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences, purposes, whether as an end or as a means, to render procreation impossible” is intrinsically evil:
Thus the innate language that expresses the total reciprocal self-giving of husband and wife is overlaid, through contraception, byt an objectively contradictory language, namely, that of not giving oneself totally to the other. This leads not only to a positive refusal to be open to life but also to a falsification of the inner truth of conjugal love, which is called upon togive itself in personal totality…The difference, both anthropological and moral, between contraception and recourse to the rhythm of the cycle…involves in the final analysis two irreconcilable concepts of the human person and of human sexuality.
Hope that helps!