I have been a professional portrait artist for over 20 years (click here for samples of portraits and other artwork), and I have been teaching art, including portraiture, for a while, too. Most of my incoming students echo a belief that I have heard stated many times over the years: "People are the hardest thing to draw."
Well, yes and no. I personally think trees are awfully hard to draw, what with all those parts sticking out every which way, but if your tree rendering is not spot-on, people might not even notice, whereas they will instinctively know that something is not quite right with a problematic rendering of a person, since we are so intimately acquainted with, well, ourselves. Portraits are even more challenging, as they not only have to look like a convincing human being, but like the specific person you are drawing.
On the other hand, we do have that head start of being so acquainted with the human figure. We see it in the mirror, and we see it around us in all its various shapes and sizes. Part of the drawing process is getting to know your subject, and with figure drawing, it's a subject we understand, as compared with, say, technical drawing. Which would you rather draw, a girl in a pink dress, or an exploded view of a toaster? Frankly, I consider anything I have to use a ruler for a challenge.
And yet, it is this seeming familiarity with the subject that can be our undoing: We may be so sure that we know what a person looks like that we have a kind of "person template" in our heads. For those of us who haven't had much training or experience, this might be a very stylized or simplified version of a person: round head, block torso, two rectangle arms, two longer rectangle legs, some sort of attempt at hands and feet. Even people who are trained artists may stick to some form of stylization. Think of ancient Egyptian art: With very few exceptions, Egyptian art followed strict rules when it came to representing people. Paintings and reliefs always showed the human figure with the head in profile, although the eyes appeared as they would from the front. Shoulders and chests faced front, but hips and legs were turned to the side. Try standing that way. Okay, now try walking that way. Uh-huh, I thought so. It's not as easy as the song makes it sound.
But that was thousands of years ago, you say. Yup. Now look at the manga (that's Japanese comic book art) and anime (same thing, animated) style of today. You have to draw everything just so, right? There are plenty of books that will teach you the rules of the style: Those large, swimming eyes; tiny mouths; nearly nonexistent noses; standard bodies. You can tell the characters apart by hairstyles and clothing, but not much else, and it's often hard even to tell gender if you don't already know the characters. And as with the ancient Egyptians, it's hard to distinguish one artist from another by their work.
If you are trying to render the human figure in a realistic style, you will have to overcome your preconception of what a person is supposed to look like. Look at drawings of people that you have done from your imagination. Do they all look like they could be the same person? I'm betting the person looks a lot like you, or at the very least is the same race and gender; maybe the same age. To counteract this, you need to stretch your boundaries: draw people of other races and ethnic backgrounds, different ages and genders. You don't have to hunt down all of these folks on some kind of weird human scavenger hunt and make them pose for you; just get some magazines that have a lot of good pictures of people (National Geographic springs immediately to mind; photography magazines will have some nice shots, as well). Draw from these photos or from live models if you can; the most important part is to use reference, not your imagination.
If you give yourself a wide assortment of people to draw, you will start to become more familiar with racial and ethnic traits, the difference in the facial and body proportions of children at different ages, and the effect of gravity on the features of the elderly. It's fascinating to see the immense range of possible faces nature makes from just two eyes, a nose and mouth, especially when you factor in differing bone structure and face shape.
So, does that mean that people are harder to draw? Actually, I would say no. We have those mental templates of everything-from cats to bicycles to hot air balloons-and we have to get past them. You might think you could draw any of those things on demand, but once you start your picture, you may find there are many questions you can't answer-do a cat's front legs bend forward or backward? Do they have whiskers anywhere other than just on their muzzles? What do the gears on a bicycle really look like? Are the spokes set in any particular pattern? What does the ballast on a hot air balloon look like?
If you get a good, clear reference photo, or if you can get an actual object or person to draw from life ("from life" just means that the thing is there in front of you, whether it's animate or inanimate, hence "still life"), you are forced to observe these details in order to include them in your drawing. To render them accurately, you must note the actual shape, size and color (or tone, if working in black and white) of various components or parts of your subject, and also the shapes, sizes and colors/tones of various components relative to one another. Don't go with what you think a bowl of oranges looks like: draw what you see; you may be surprised at all the variation from orange to orange, as well as within the bowl, the tablecloth, and whatever else you have in the setup.
You have to use this same method of analysis on a human sitter as well as on a bowl of fruit, so technically, it's really just as easy to draw people as to draw anything else accurately. Feel better now?
My original work "In the Flow", charcoal on pastel paper
Here is another article you may enjoy about figure drawing: http://allsortsartbyali.blogspot.com/2015/03/figure-drawing-101-wacky-secrets-of.html