Sometimes it's just a minor adjustment that you need to do to see massive improvement in your art. If you ever tried to draw hair, eyelashes, or any sort of dynamic shape, you probably had to do it more than once and had to go over your lines again and again to get it to look right.
The problem with this is that it will stiffen your lines and deprive your work from looking lively and organic. The following example is going to be very useful in deepening your understanding of this.
Let's see it together!
Look at the first set of lines and try to notice the difference between them and their counterparts on the right. The first set on the left are all of the same length and width; they're almost identical.
Look at the next group of lines. Again! Just like the first set.
There is no variation in their direction nor in their size, which simply looks odd and wrong.
Nothing robs your work from its glamour more than lines like this, especially when you're depicting organic objects. The funny thing is that this also applies for nature and its relatively still elements.
Contemplate the following scene and try to notice the shape patterns of the different components.