Sincerity and honesty are the building materials of a good and strong relationship. On the contrary, secrets and lies will dig a grave for your relationships.
When you keep things related to the relationship as a secret to your own, you are not being sincere with your partner nor with your relationship. Or maybe, for you, the relationship isn’t close enough to share everything. It is this skepticism that will disrupt your relationship.
In particular, lies are unacceptable and forgivable in a relationship. That you’re lying means you don’t respect your partner. That means you let your partner misunderstand you.
You don’t let them know who you really are, what’s happening between you two, so how can they love you? How can one build a relationship based on a lie?
And no matter how carefully you hide it, sooner or later the truth will come out. The relationship will be broken up, and your partner is forced to leave. You will have nothing left.