I've said several things - I kept my name so it's easier at school, I'll change it when I have kids, etc. People do it for a variety of reasons - some for tradition, some for ease once children are born, some in order to honor a family they feel did a lot for them. Look it up. More men are taking women's last names than before, many stating that their in-laws helped them through hard times or something similar. Other couples have been combining their names, breaking the hyphenation rule and even blending them into one. A well-known example is Mayor Villaraigosa. He and his wife combined their names.
Aside from those innocently asking for my new name I've been bothered already by my university's financial aid office. They asked me several times to submit my legal name change and my new Social Security card. I got so exasperated I finally went to the office in person. I explained to the woman at the desk I hadn't changed my name. She smiled and very genuinely asked if it stayed the same because, by coincidence, my husband had the same last name as me.
My brain jumped to, "Is this really the only way she could believe I kept my name?" I told her it was too complicated to change it at school and that maybe I'd change it when we have kids. There I went, spitting out the same old excuse. She seemed appeased by this though. She made me submit a handwritten and signed/dated statement that I had not changed my name then sent me on my merry way.
I'm keeping my name because I feel that I've accomplished enough under my current identity that I want to keep it as-is. Also, it was my father's last name and he passed it down to me. My husband is a great part of my life but my dad dedicated eighteen years to raising me. Maybe when Felix has put in his 18 years I'll reconsider.