From personal experience, the term 'wear your heart on your sleeve' symbolizes empathy and sympathy for another person as the most deepest of emotions being felt at the time. It also is an expression used for somebody that puts other people before themselves, no matter what situation they may face in their own life. It is for a person that understands another in the way that a lot of people wish they could find and feel. 'Wear your heart on your sleeve' is a quote that is used for people that do everything they possibly can to support another person in a life or death situation, and are not concerned about what they are receiving as a reward for their actions.
For example; you save somebody from committing suicide and do what you can to make sure the person stays out of harms way, even if it is deteriorating your mental and emotional health to do so. (This is an extreme example, one that most people cannot follow through with without completely shattering).