the heart has its reasons which reason knows nothing of?
A very nice statement, I really like it, hope I understand it the way it should be understood :)
Here you have two meanings of the word "reason":
in the first part of the sentence (before "which") "reason" means a cause, intention,
in the second part "reason" means rationality, common sense, logical thinking,
and now an example:
you see the most beautiful girl/boy (depends on who you prefer :)) and you don`t know this person but you fall in love (at first sight), you heart says - I love her/him - your heart has its own reasons to make you fall in love, it doesn`t even care whether this person would be good to/for you, you`re in love and there is no way out :)
But after a while you may think... THINK (you use your brain, mind, reasoning), hey, it`s stupid, I don`t know this person and I`ve fallen in love with her/him - it`s stupid-you mind tells you so.
So here is the point: your heart has its own reasons-intentions (to fall in love for example) but you mind/logical thinking=reason doesn`t understand them, doesn`t know anything about the reasons of your heart, you reason doesn`t know why you`ve fallen in love.
I hope it`s ok