Your relationship suffers whether you think your partner is keeping one big secret from you or is just acting secretive in general. Of course, people do not tell each other everything about themselves right at the start and only gradually open up. However, research published in the "European Journal of Social Psychology" in 2012 showed that when one partner holds back and conceals things, the other partner loses trust and may eventually self-disclose less as well. Emotional intimacy is reduced, and this can turn into a vicious cycle that is difficult to stop.
Be emotionally available. Perhaps you are the one who finds it difficult, possibly even scary, to open up and share things about yourself to others. If this is so, you may be seeing your partner turning secretive in response to your own difficulties in trusting him with deeper knowledge about yourself.
Tell your partner how you feel. In a nonaccusatory fashion, tell your partner that you see him as being secretive and give some examples of what leads you to that conclusion. Then say how that makes you feel, perhaps something like: “When I think you are holding back from telling me things about yourself, I feel as if a wall went up between us. I love you and want to feel closer.”
Respond in a positive manner when your partner does tell you something, however small. In the 2011 article “When Partner Caring Leads to Sharing,” researchers report that people open up when their attempts at communication receive responses that are “understanding, caring and validating." This might start the relationship down a new path of increasing trust and self-disclosure.
Give it time and be willing to discuss the issue repeatedly. Social scientist Andreas Wismeijer wrote in "Emotional Regulation and Well-Being" that people who keep secrets may have low self-esteem and hide facts about their past or their personalities as a way of protecting themselves. Alternatively, if your partner has grown more secretive over time, it may indicate relationship problems that need attention. In both cases, opening up will grow slowly as you build or rebuild trust.