When you're involved in a romantic relationship, your suspicions and insecurities often lead you to try to control your lover's feelings. The idea that they may be thinking things that you have no clue about scares you at a deep level. You're simultaneously intrigued by and frightened of the unknown; what better way to turn the unknown into a known quantity than by controlling it yourself?
If you can force your lover to feel a certain way, at least you'll know what that feeling is -- that's the idea, anyway. It doesn't really work that way, and a perfectly good relationship could be flushed down the tubes if you indulge too often in manipulative tactics. What's wrong with letting your lover keep some things private? It's scary to realize that you don't know every dark corner of their soul, but that's just one of your challenges in life -- not to get angry when faced with something unexpected or inscrutable from a lover. In the best of circumstances, your sweetie will constantly surprise you with their unique feelings and responses; that's what it is to be involved with an individual. Your task is to learn to appreciate their individuality instead of taking it as a threat.