According to a study I saw on the NPR website, parents are not happier than the childless, but other studies and convential wisdom hold that married men are happier than single ones.
I believe your mother is pressuring you because (depending on the culture you live in) marriage and parenthood increase a person’s status. Especially for people of her generation, having a spouse and family equals success. Many parents believe that their children’s success reflects well on them, and that their children’s social status influences their own. Furthermore, if all her children have many grandchildren, that increases her status among her agemates. Don’t believe me? If so, listen in on a group of older women sometime. What are they most likely to talk about. Aside from their hobbies, most of them will brag about their kids and how many grandkids they have.
Of course, just because she wants you to be fruitful and multiply, it doesn’t mean you have to. She’s already had a chance to live her life. Now it’s time for you to live yours, and that means deciding for yourself whether you want to live the single life or take the family path. If your mother continues to pressure you, say, “I’ve got an idea. Why don’t you become a foster parent or sponsor a child in a third world country? There are lots of needy kids out there.” Maybe then she’ll take the hint.