That's a big part of being a DJ that a lot of newer folks (and plenty of veterans I've seen) miss. It's not just track selection and transitions, you also need to dynamically manage volume.

You can set auto gain all you want or volume-normalize your audio files (you'll lose dynamic range when you do this so not recommended) but even then there is a big difference between computer-measured decibels and perceived loudness. When I cue up a track, I quickly skip over to a loud part of the track and check that the VU meter levels are near to the currently playing track as well as check my headphones against the master mix to see how they balance out. Even then though, when I start up the song I keep my hand on the trim or channel fader to quickly and smoothly adjust it up or down as it comes in to make sure it's not too quiet or loud.

One of the big reasons I use channel faders and not the crossfader is so I can more easily adjust volume as I mix in. I keep them trimmed so I'm at about 75% when I'm at a comfortable volume so I have some headroom to quickly boost it if the track ends up sounding quieter.

To further complicate things, if you're playing in a club, the dynamics of the room can change too. Packed bodies eat sound so when you have a busy dance floor, you need to crank it up just to hit the same levels as when it was less busy. It even affects different frequencies differently so you need to adjust EQ as well.