Why would someone feel sorry about me ?

One of my school mate messaged me today on Instagram. She was randomly asking me things about my life . She asked for long I’ve been married .. I said 6 yrs .. then she asked me do you have kids I said no .. she said oh I’m really sorry about that . I said why are you sorry about it ? I’m happy in my life and that’s not the only aim of my life . ( but I felt really bad when said that, why do people do that ? When they don’t know other person struggle how can they don’t ? Sone people even never have kids all their life, not by choice but by destiny so are they not allowed to live happily ? Feeling really bad.