No one has ever been able to tell me what “wearing your heart on your sleeve” means. I’ve found numerouse websites devoted to cliches but none had this one. Glad I’ve found TSD!!!
Further to MeBuckner’s reply we have this from Brewer:
To wear one’s heart upon one’s sleeve: To expose one’s secret thoughts or intentions to general notice. To show plainly one’s feelings. The reference is to the custom of tying one’s lady’s favour to one’s sleeve, and thus exposing the secret of the heart.
Minor hijack-is there a known instance of this which antedates Othello? I had always assumed that Shakespeare coined the phrase, but it just occurred to me that this was pure speculation on my part …
It means to be very sensitive and easily hurt in relationships because you expose yourself too much. It also means that it is obvious that you care for a person, even if the relationship is still too young for them to care much back in return. To wear your heart on your sleeve is to repeatedly get easily hurt in relationships.
Someone who wears their heart on their sleeve is just asking for one heartless @%#$ after another to just rip it right off and STOMP ON IT REPEATEDLY.
(Not that I know anyone this ever happened to, of course.) 
I disagree, this is way too specific–there are plenty of ways you can “wear one’s heart on one’s sleeve” that have absolutely nothing to do with relationships. This phrase, as MEBuckner and Nostradamus noted, has traditionally meant that one is particularly demonstrative and/or transparent in the way one externalizes emotion. I have never heard it used to specifically refer to how “vulnerable” somebody is, and to be externally emotional does not necessarily mean one is overly-sensitive. This term is not necessarily pejorative, though your definition would have it be.
I don’t have a cite, but I remember being taught in grade school (so of course it MUST be true) that “wearing your heart on your sleeve” harkens back to the beginning of sending valentines to one another - it was not done more or less randomly, as in grade school when we made valentines for everyone in our class - it really was supposed to mean something. If a fella got a valentine from some gal he really wanted one from, he literally wore it on his sleeve. Hence, the reference.
I wouldn’t be surprised if someone finds a cite to blow this explanation out of the water, but I do remember reading it…