I thought having twins would be good and wanted ivf. But doctor said no. Why?

Increased risks. Multiple births increase risks for both mother and child because the uterus is designed really for only one baby. The more babies crowded in there, the more problems so that is why your doctor discouraged you from tampering with mother nature. She sometimes knows best.
IVF twins. You really have to ask your doctor for his answer, but no reputable fertilty center will offer ivf purely for teh purpose of having twins.