Pain.
Whether physical or emotional, it comes in a lot of shapes and forms - it may be treatable or untouchable or even unresolvable, depending on where it comes from. People may ask you to rate it on a scale from one to ten when in a lot of situations the numerical rating isn't worth a damn.
Some of us walk around bearing it with a smile on their face. Others are crippled by it and are reduced to a human tear drop by it. A school of thought believes that the absence of pain is the absence of growth. Another believes that a lack of pain is a sign of disease.
Sometimes it's a sign that something is changing. Sometimes it's a sign that something is wrong. Regardless of why it's there, it's usually there for a reason.