It's not uncommon for Dungeon Masters to make a large, sprawling layout for their dungeon only to look it over and think "...That looks awful for some reason".
First, filling the dungeon with creatures and events will help get rid of that. But even then, some dungeons are actually poorly made. Here are a few things you can do to make your dungeons better.

1. Not every room is a square and rooms should look different in general. Any ol' shape is acceptable, so long as it fits some type of reason.
2. The theme (see previous post) will help you determine how the dungeon should look, even on a layout level.
3. What is the purpose of each room? Is it a break room? Is it for training? Eating? Study? Knowing what a room is for will help you pick a shape, its contents, and a good description.
4. Consider the Z axis. It will add a lot of visual interest to make the place have differing height levels, either through stairs, slopes, cliffs, or that sort of thing.
5. Don't let rooms be empty voids. In very few cases will this be appropriate, so put some scenery in those rooms.

Beyond those, unfortunately, practice is the best way to get better at making dungeons maps. Sad, I know, but it's going to be a very commonly recurring theme I'm afraid.