You’ve probably had this experience: You’re working on something with a great deal of concentration and for a moment, your attention is diverted. When you try to pick up where you left off, somehow, everything seems smaller and not in the same perspective as it was before you looked away. This is a natural function of the mind; to amplify something we concentrate on. In mixing, the same phenomenon occurs. A track you have soloed and focused on will sound louder to you (than it really is) once you put it back in the mix. This is particularly true with vocals. Once you’ve learned the words and know the performance intimately, the vocal track will tend to sound louder to you in the overall mix than it will to someone who is just hearing it for the first time. Keep this in mind when you mix so that you’re not tempted to reduce the level of the track, burying it in the mix. The best solution is to wait a day (if time permits) and come back to the mix. You’ll find that your perspective has changed.
Speaking of buried vocals, some bands insist on having vocals buried in a mix so that it will cause the listener to turn up the volume and play the song louder. Another reason to keep vocals lower in the mix is so that they won’t obscure other rhythmic activity in the song, making for a more balanced sound overall. The Rolling Stones and Pink Floyd use this technique.