In the Western world, and especially in countries where commercial surrogacy is not a legal possibility, most surrogate mothers have altruistic reasons for becoming a gestational surrogate. Most already have their own children, and want to help other people realize their dreams of becoming parents, too. Whether a woman is considering carry a baby for a friend or relative, or for someone they didn't know before the surrogacy journey, they tend to see surrogacy as a beautiful gift. There are plenty of women who enjoy being pregnant and giving birth a lot, and would like to do it again but feel their own family is already complete. For those women, becoming a gestational surrogate may seem like a mutual gift, rather than a sacrifice.
Surrogacy is not easy, however. Gestational surrogates often need to take injectable fertility medications to prepare for embryo transfer, and can face the same pregnancy complications that can befall any pregnant mom. By being pregnant for another person or couple, a surrogate also risks having to undergo a cesarean section. There are, of course, surrogate mothers whose sole motivation is financial gain. The ethical questions raised by this are the exact reason commercial surrogacy is not legal in most jurisdictions. Those who end up being a surrogate to make money often discover that "renting out their uterus" is anything but easy money. Also look at surrogacy costs in the US and interested in surrogacy in India?