I've been following Memorising Music (sic: UK spelling), a blog about memorizing (sic: US spelling) music as told through interviews, articles, and personal experience.
In this post on "earworms," the author discusses the earworm phenomenon and goes on to suggest that, because just about all of us experience earworms, "we are all capable of musical memory...[and that]...some music is naturally easier to memorize."
Indeed! Every so often, I encounter someone who says that he or she is "tone deaf." While tone deafness is an actual condition in which one can not hear frequency differences, it does not describe the phenomenon of being unable to match pitch in singing.
(Not being able to match pitch is something that can be fixed through basic singing lessons. I usually find it among people who didn't grow up in a family that sang songs together--an increasingly common condition it seems!)
So, before you say that you "can't memorize," consider that the guy who claims he's "tone deaf" is just one negative self-image behind you. Both matching pitch and memorizing are skills that can be learned.