Why the kick sits at the center of level hierarchy
The kick drum is almost always the loudest element in a mix because it carries the frequency range that listeners perceive as power and forward movement. Placing kick at around -6 dB to -4 dB on your master meter gives it undisputed presence, with everything else layered underneath or around it.
Bass frequencies are felt more than heard, so bass instruments (synth bass, sub) live just below the kick in prominence. Both occupy the low end, so their levels must be carefully balanced to prevent muddiness. A bass that's too hot will compete for the same psychoacoustic real estate as the kick, resulting in a blurry, undefined low end.
Vocals, snare, and the mid-level structure
Vocals typically sit just below or at the same level as the kick, making them the second anchor point. Listeners expect to hear the human voice clearly in a mix, so erring slightly hot on vocals is safer than burying them.
Snare and lead elements live 2-4 dB below the kick, giving them punch without overshadowing the fundamental rhythm. The snare's bright transient sits in the mix even at lower levels because its presence in the upper midrange cuts through naturally. This level stack is not a rule but a listening reference; adjustments for genre and song arrangement are always necessary.