The worst thing a person can do to another is offer conditional love. I submit that it is the cause of much sorrow and darkness. Perhaps the greatest cause. I would rather be hated than be the recipient of conditional love. To say that affection will only come if the object of affection does what the giver wants is a sentence of living death. You think I am being too extreme? Well, let's partake of an example then.
My favorite tv show is Smallville (I'm sure you all knew that). The relationship on Smallville that most interests me is the one between Lex Luthor and his father Lionel Luthor. It portrays the results of conditional love. I know, this is fictional, but stories can often illustrate to us what we find confusing in real life. When we first meet Lex he is the subject of his father's will. Lex is strong willed and emotional. He desires to be a good person and the audience cannot help but like him. Lionel is ruthless in his treatment of Lex. He places Lex in charge of a Luthor Corp plant and demands Lex achieve perfection in all his business dealings. Lex tries passive aggressive strategies to bypass his father's inflexible standards. Somehow Lionel is always one step ahead of Lex. It's like a horrible game between father and son. Nothing Lex does is good enough for Lionel. Lionel pits Lex against himself just to teach Lex how to be strong. We get a few flashback moments between Lex and Lionel. What we learn is that Lionel has always told Lex that he is weak and pathetic. That's how he tries to be a good father to Lex. Even when Lex does the right thing Lionel can find some mistake in it. Lionel himself is committing the greatest mistake a father can commit: conditional love. It is a mistake that teaches Lex he must be a perfect son or forever live ashamed and angry. Later in the series Lionel turns his life around and tries to reach out to his son. He is eventually able to offer the unconditional love that Lex never knew. But then it is too late. Lex is a bitter young man who refuses to believe his father could love him. Lex has taken conditional love to the next level. He distrusts everyone around him. He drives away even his closest friends because he constantly manipulates them in order to keep their friendships. And they tire of it. He is a lonely, sad, dark, obsessive, manipulative person.
So I repeat, the worst thing you can do to another human being is offer love based on their performance of your standards or wishes.
At the same time unconditional love can be taken to an extreme or used in an unhealthy manner. It's kind of like the battered wife principle: don't allow the abuser back into your life without proof of full repentance because that's unhealthy. Don't allow someone to take advantage of you just because you love them. Limits must exist in order for unconditional love to work. Those limits, however, should never be more important than the love we have for those around us. First offer the love, then the limits. If you go the other way round the person will probably never realize you care.