Starting in the ’90s, artists and bands wanted their release to be the loudest release in a listener’s multi-CD changer, so that it would stand out. Thus, the “loudness wars” began, in which every album fought to be louder than everyone else’s. This was accomplished by using loudness maximizers or limiters to raise the average levels of the songs by crushing the highs and the lows, resulting in very loud recordings, but no dynamics. So ultimately, the loser of the war was us, as nearly every album ended up sounding dynamically flat and sonically fatiguing.
Over the last few years, however, that seems to finally be changing. Artists seem to have realized that their music (and their fans) were suffering, and have pulled back from the brink, allowing more dynamics in and not limiting tracks to death. What this means in a practical sense is that when you master your songs — whether you do it yourself or take it to a pro — go ahead and raise the overall level until the peaks are as close to 0dB as you want, but not so much that you overly reduce or squash the dynamics of your recordings. Let them breathe, even if that means some passages are softer than others. Your music will sound more engaging, and your audience will thank you.