The gemara tells us that when R' Akiva was being killed and his flesh was being combed, he was accepting upon himself ol malchus shamayim and he said to his students כל ימי הייתי מצטער על פסוק זה..., all my days I was troubled by this pasuk בכל נפשך, with all your soul, which means even if he takes your soul. I said to myself when will the opportunity come that I can fulfill this. Now that it has come into my hands, should I not fulfill it? What is meant by “All my days I was troubled by this pasuk”?
R' Tzvi Hirsch Meisles explains that when one recites shema, he should have in mind that if he is ever put to the ultimate test, he will be moser nefesh and give his life al kidush Hashem.
Now, when R' Akiva said this pasuk he imagined he was giving up his life al kidush Hashem. So R' Akiva felt the pain of that moment.
R' Akiva was essentially saying I know that if I am ever called upon to surrender my life for Hashem, I can do it when I say shema because I have lived through that experience every day when I say shema.
This is what is meant in כל ימי הייתי מצטער על פסוק זה: I lived that experience of pain, of the process of dying al kidush Hashem, each time when I said this pasuk. And now, having been granted the actual opportunity to give up my life while reciting shema, how can I not fulfill it, that which I anticipated my entire life?
Brachos 61b.
Devarim 6:5.
See Mishna Brura 61:3.
