As a start, try to identify the individual, whom you can't resist from making him/her sad if he/she is happy and vice versa. In doing so, you might develop some understanding about yourself in terms of how you are different to them and/or do they remind you of a person, whom you hate or like and hence behave in a certain way. If problems persist and you really want to get rid of it, please consult psychologist for help. Best wishes.