Its hard to believe that no matter how much you may think you know someone the things they are capable of will still continue to shock you. Being a person that falls head over heels in love is an amazing thing however if you get involved with the wrong person it can be terribly detrimental to both parties involved.
I guess the best thing you can learn is no matter what your heart tells you always keep your eyes open. I have found that "love really is blind" so make sure you always take into account the feelings of those around you because nine times out of ten they will see things you are incapable of seeing.
Its important to remember never let yourself become jaded but at the same time always fall in love leading with your head.