Cleaving Through Total Bittul
ובמדרש בפ' שמע כו' ואהבת
In the Midrash, in the passage of Shema, on the verse 'And you shall love' (Devarim 6:5), and so forth.
The Sfas Emes opens by citing the Midrash on the Shema passage and the words 'And you shall love,' setting the stage for his discussion of the Shema and ahavas Hashem.
מי לי בשמים כו' והענין הוא כי שורשן של נשמת ישראל בשמים הוא באחדות ה' כמ"ש ה' אלהינו ה' אחד
'Whom have I in heaven [but You]' (Tehillim 73:25) — the meaning of this is that the root of the neshamos of Bnei Yisrael is in heaven, bound up in the Oneness of Hashem, as it says, 'Hashem is our God, Hashem is One' (Devarim 6:4).
The neshamah of every Yid is rooted on high in the absolute Oneness of Hashem, which is exactly what we declare when we say 'Hashem is One.'
אך להתדבק בזה האחדות בעוה"ז הוא ע"י ואהבת בכל לבבך נפשך מאודך
However, to cleave to this Oneness while in this world comes about through 'And you shall love Hashem your God with all your heart, with all your soul, and with all your might' (Devarim 6:5).
While we are down here in this world, the way to actually connect to that Oneness is by loving Hashem completely — with heart, soul, and all our resources.
אז והיו הדברים האלה כו' הנאמרים בפ' שמע על לבבך
Then, 'And these words [which I command you this day] shall be' (Devarim 6:6), the words stated in the passage of Shema, 'upon your heart.'
Once that love is real, the words of Shema can truly rest 'upon your heart' and take hold within a person.
והוא ע"י הביטול אליו ית'
And this is accomplished through bittul — nullifying oneself before Him, Yisbarach.
The mechanism that makes this connection possible is bittul — completely nullifying one's own self before Hashem.
ולכן חכמים הגידו למסור נפש באחד שא"א לבוא להאחדות בלתי מס"נ
It is for this reason that the Chachamim taught that one must give over his soul upon [reciting the word] 'echad,' for it is impossible to arrive at the Oneness without mesirus nefesh.
This is why our Sages taught that a person should be ready to give up his very life at the word 'echad,' since the Oneness can only be reached through total mesirus nefesh.
וז"ש ועמך לא חפצתי בארץ לבטל כל הרצונות אליו ית' אז יכולין להתדבק בשורשו שבשמים
This is the meaning of 'And together with You I desire nothing on earth' (Tehillim 73:25) — to nullify all of one's desires before Him, Yisbarach; then one is able to cleave to his root in heaven.
When a Yid declares he desires nothing on earth apart from Hashem, he nullifies all his personal wants, and only then can his neshamah cleave to its source in heaven.
והוא העדות של בנ"י שנאמר אתה הראת לדעת כו' כמ"ש לעיל:
And this is the testimony of Bnei Yisrael, as it says, 'You have been shown, in order to know' (Devarim 4:35), as explained above.
This complete surrender is itself the testimony of Bnei Yisrael, who were shown the truth of Hashem's Oneness 'in order to know' it, as the Sfas Emes explained earlier.
Summary: The Sfas Emes explains that the root of every Yid's neshamah lies in heaven, bound up in the absolute Oneness of Hashem that we proclaim with 'Hashem is One.' To connect to that Oneness while living in this world, a person must fulfill 'And you shall love Hashem' with all his heart, soul, and might — and this is achieved through bittul, nullifying himself entirely before Hashem. For this reason the Chachamim taught that one must be ready to give over his soul at the word 'echad,' since the Oneness cannot be reached without mesirus nefesh. When a Yid desires nothing on earth but Hashem and nullifies all his personal wills, his neshamah can cleave to its root on high. This very surrender is the testimony of Bnei Yisrael, who were shown Hashem's Oneness in order to know it.