Nope.
Many reasons; homones, poor family situation, fear of being a mother, childbirth trauma, jealousy... I doubt I could possibly name them all.
Brooke Shields just wrote an interesting account of her experience with post-partum depression (PPD). She attributed most of it to fear and a complicated relationship with her own mother. Anywhere from 6-15% of women are known to experience PPD, with even more experiencing a perfectly normal hormonal let-down known as "the baby blues", but most of these women (including Brooke Shields) survive the initial depression and go on to be happy mamas.
Then there are the mothers who are never happy. These are the ones that tend to abuse and neglect their children, very sad but very real considering that about 2 million cases of child abuse and neglect are reported in the US each year.
Typically, those women who want children before they become pregnant *do* welcome their babies with joy, if not right away, then at least soon after they are born. For those women who did not want children, some will recognize the wonderful gift they have been given, others never will...