In short it is to fullfill the commandment to love Hashem with all your soul
See the second source of the excellent answer above
Brings that Rabbi Akiva explains that to love Hashem
"With all thy soul" means even if He takes away thy soul.
That the gemoro bings an example of this by going against the law to keep Hashem's commandments/will (when you love someone you want to be close to them and by doing their will you become close) and continues that
"When R`Akiba was taken out for execution, it was the hour for the recital of the Shema', and while they combed his flesh with iron combs, he was accepting upon himself the kingship of heaven.
We know from the mishna that the recital of the first chapter of shma is the accepting of the kingship of heaven.
So from this it seems the love of Hashem is even more revealed by fulfilling the spesific commandment/will of saying the shma (3 chapters) and spesificly the accepting of the kingship of heaven
chaddidus seems to me to be mostly about love and fear of Hashem.
To love of Hashem (doing his will) is required even when Hashem is killing you ("even if He takes away thy soul.")
So by all mitzvot by having in mind (potential self sacrifice) that you will do His will even if He takes your life
But spesificly the mitzva of saying the shma (on the right time) and accepting of the kingship of heaven to all the heavens and the 4 directions of the earth (berachot 13b) by having in mind that you are ready to do this even when Hashem is killing you (as Rabbi Akiva did), you are filling the mitzva of loving Hashem to the fullest and biggest extent (possible in your situation (when your live is not being threatened))