As an art student, I got really drilled about making sure I stayed away from just drawing floating heads. I had a pretty cool illustration professor (looking at you Mr. Rea) who had us keep a real sketch book for class. We had to draw A LOT. More than I was used to, for sure, at the time. I think we had to draw a sketch a day and each week we'd have to hand over our sketch books to be checked as part of our overall grade for the class. He rarely critiqued the sketch books heavily, that wasn't the point of them, but he did have a particular grudge against floating heads. It's a common issue with many artists - incomplete drawings.
Thanks to his light pushing to stop doing that, I started to finish the sketches as much as I could. Heads always had at least shoulders attached to it, backgrounds started to happen, though very primitive and simple. One of my dreams has always been to illustrate for books. But I know that's not going to happen without at least a basic understanding of backgrounds and landscapes. So whenever you see a sketch of mine that has a background of some kind, get excited with me because it's a big deal!
Note: it is ABSOLUTELY ok to have sketches like the one above. That's why it's a sketch. If you want to just draw eyes or lips or hands or whatever from here till the end of time, have at it! Just remember your teachers want you to grow and try different things so they're going to pressure you to constantly step outside of your comfort zone in your creations. Don't be upset at them, that's their job. Your job is to learn from them and decide for yourself what practices you want to keep or discard. There are bad teachers out there so don't take it all as gospel. Use your head.
With love - M