Depression
Many people seem to forget what isn't always visible- and who can blame them? But it's always going to be there; even if you CAN see it or not... It's something that'll never go away. Hurt, depression, anxiety, loneliness- things that aren't physical but emotional. People don't know how to cope with the all these feelings and think that there isn't a way out.
There is. And it starts with you. If you see someone who seems down and blue, make them smile at you! Make their frown turn upside down; be there for them when they think others are not! A simple "hi" or "how are you?" Is sometimes all they need to realize that people do indeed notice them! Encourage the joy and happiness you have unto people around you and you'll see that even you will be happier!