The first is that in general, food prepared by other people always tastes better. This is why one can be an excellent home cook but still enjoy eating out. Having someone else take on the labor and love of preparing a meal is a real treat and definitely 'seasons the sauce.'
Similarly, sometimes when I cook, by the time I sit down to eat my dish I'm kind of done with it. Between tasting it for seasonings and doneness and inhaling all the aromas of cooking, I don't get that clean, first-bite thrill when I actually eat it. There are no surprises in flavor or texture. So while I can be satisfied that it tastes good or even great, it's more of a confirmation than a discovery. (This explanation works for complex cooked dishes but perhaps not for a simple sandwich.)
Finally, the main reason why I think my mother's sandwich tastes better is quite simply because my mother made it. A sandwich is a classic made-by-mom treat and even when as an adult, there's still something special about eating a sandwich made by your mother. In this case, the sauce is being seasoned by love or at least it's that way with my mum!
(Image: Modern Reject)