Using biro for me is the most spontaneous and surprising way to make art. I sometimes start doodling and then find that my doodle turned into a little art project. I truly don't expect these things to happen - after all, I only used a pen, right?
The humble pen is probably the least appreciated in the world of art. You can't erase or undo what you've done. You can't even cover it up. I suppose you can always use liquid paper (correction fluid, or Tipp-ex depending on which country your from) to fix your boo boo, but let's be honest here. That's just going to make it worse.
I don't see the mighty pen as such. Other art mediums require more effort to prepare, whereas pens are normally always around and ready to use. It won't dry up when I leave it (within reasonable time, i.e. not ten years) and it won't ruin my tools if I don't wash it properly like some paints (acrylic paints!). I can come back to my art with very little thought and therefore not much time to tempt myself to watch anime or game instead.
Since I don't expect much from pens, I don't expect much from my art. The less expectation I have on myself, the happier I am. If I unknowingly and happily created something amazing, that's just a bonus.
There are so many different types of art mediums available, that I find it hard to remain faithful to a specific one. Admittedly, I do have my favourites such as the humble pen and acrylic paints, but I'm never scared to venture into watercolour, charcoal and digital art. After using a specific medium for some time, I find it refreshing to use something different. For instance, I like the shift from controlled and calculated strokes with a biro, to free flowing strokes with a paintbrush.