It really has to do how airlines roser their trips and seniority levels. With DL and other US carriers older FA's get first dibs on flights and choose the longest possible flights, hence why other than language speakers, you are bound to get FA's of the same seniority regularly on the same types of flights and if you fly them very often, you'll get to know them, hell, I know many by name, they know me, and we even talk about our families, other FA's etc, from the top of my head I can say that there was one particular DL FA that I had on no less than 20 trips, I also know that she just retired!. - as opposed to BA for example that does not have this same system and retires everyone by 55. That being said, despite the fact that I fly BA less than DL long haul, I have had the same FA's on several trips but not that often.