Why does uterus have to be prepared every month for fertilization? I'm just curious why it has to happen every month when the uterus could have been prepared from the beginning and kept that way throughout life or till a certain point. But why make and break it every month? (The breaking of this preparation is the cause of menstrual pain, which is what I'm worried about).

A quick search through the Internet, as well as any textbook, will reveal that the lining of the uterus (endometrium) created thickened during every menstruation cycle is rich in nutrients, and requires a constant blood supply, in order to accommodate a fetus.

Therefore, maintaining the lining would be more costly to the body than rebuilding it every month; in fact, as part of the lining is reabsorbed, rather than destroyed and expelled, thus conserving energy and nutrients. The reason a part of the endometrium is not reabsorbed is its thickness, which prevents its total reabsorption.

Also, the menstrual pain is, in fact, caused by the breaking down of the endometrium, although not directly, but by muscular contractions (cramps) and the contraction of blood vessels, which happen to prevent blood loss and help with the expelling of the endometrium.