The Smithsonian offers some good explanations:
Here's the link: www.smithsonianmag.com/arts-culture/the-origins-of-wearing-y
Where did the term 'wearing your heart on your sleeve' originate from?
Well , from first hand experience,
I was a sucker more then once,
And if you fall easily in love .
Or care to much,
You "wear your heart in your sleeve "
Not sure what sucker became the but end of this. I know I was a few times,