"And G-d spoke to Moses after the death of Aaron's two sons who drew close [to bring an unauthorized offering] before G-d and died." (Lev. 16:1)
Rabbi Yisroel of Tchortkov posed this problem. The Torah tells us, "Only you who are attached to G-d the Almighty, remain alive today." (Deut. 4:4) G-d is the source of life and one who draws close to G-d, attaches himself to the source of life. How could it be that Nadav and Avihu, "who drew close to G-d", died. The purpose of a Jew is to refine and repair every aspect of his soul, until each one is working together in harmony…
It is written, "The beginning of wisdom is the fear of G-d" (Psalms 111:10). One must also, "Love the L-rd your G-d" (Deut. 6:5). The purpose of a Jew is to refine and repair every aspect of his soul, until each one is working together in harmony, each quality in its proper proportion. If one would serve G-d only through love or chesed, he couldn't survive. The chesed of G-d is boundless and so great that no one is capable of receiving so much without perishing. Similarly, if one served G-d only through the quality of fear, or gevura, he couldn't survive either. G-d is so awesome, who could stand before him? Therefore these two qualities must work together in proper harmony.
Nadav and Avihu drew close to G-d (represented by the name Havayah, the aspect of G-d's chesed), with only one quality - that of chesed - and therefore couldn't survive. "Only you who are attached to G-d ["Havayah", representing chesed] the Almighty ["Elo-him",representing gevura] remain alive today" - only those who serve G-d with both qualities in harmony will endure.
[Based on Ohr Yisroel, parashat Achrei Mot 5647;
First published in B'Ohel Hatzadikim, Acharei 5759]