A surprisingly common problem people have is when their neutral or resting facial expression sends the wrong message about how they're feeling. They may be told they often look angry, sad, worried, or expressionless. Whatever the misleading facial expression is, it can cause a few problems. It can give people the wrong idea about you, and perhaps cause them to avoid you. It can lead to a lot of annoying, intrusive comments, and if someone is constantly being told about the false message their face gives off, they may become quite self-conscious about it.
A few factors can cause someone's face to portray a mood that's not in line with their actual mental state. Regarding natural facial features, slanting eyebrows, a heavy brow, a creased forehead, deep set or squinting eyes, or a naturally intense gaze can all make someone look angrier than they are. Large eyes can make people look more worried and startled, while naturally downturned lips can cause a person to look like they're frowning. Furthermore, someone may be quicker to conclude a person is showing a certain emotion if it matches up with a stereotype they fit.
In some cases, it is a self-protective habit. Some people adopt an angry or intimidating demeanor to protect themselves, but then can't break the habit when it's no longer useful. A woman with social anxiety may try to look unfriendly and unapproachable so she can ward off uncomfortable interactions. A guy with a history of being picked on may want to look tough at all times so no one tries to mess with him.
Sometimes someone actually is feeling an upsetting emotion, and it's showing on their face. People think they feel okay, but if you were to really examine their thoughts and mood, you'd find that's not entirely accurate. Depending on the person's face, their expression tends to look angry or anxious when they're generally feeling tense, preoccupied, and not perfectly happy. For example, someone who feels nervous and awkward at a party may outwardly come across as grouchy.
Alternatively, their facial expression may match the emotion they're feeling, but their actual mood isn't as intense as their face indicates. For example, someone at work may be told they look angry. At most they're slightly bored and irritated. Or someone may be told they look sad while they're out. They're not about to cry or anything, but they've had enough for the night and they're drained.
Finally, their expression and the emotion it signals actually match up, but they're not completely in touch with how they're feeling. Some people do spend a lot of their time feeling grumpy, stewing about all the ways they've been wronged. But these thought patterns are so frequent and automatic they don't realize when they're having them. Similarly, someone may have so much background sad or anxious noise in their brains that it becomes the new normal and they don't think of themselves as always feeling that way.