Simple. Usually the higher version of anything, i.e., fear, bittul, love, etc., is essential. That means it shakes the person to their core and is usually experienced only by tzaddikim. The reason for this is that a person cannot transform themselves into something greater than themselves. They don’t have the “materials.”
“Lower” anything means it is:
1. Externally motivated (chitzonius’dik).
3. Characterized by beinonim.
4. Limited and by limited beings and therefore:
5. High-quality. “The greatness of light that emerges from darkness; of wisdom from foolishness.”