Nobody wants to appear vulnerable, and sometimes that can go double for the person we are closest with. While we may not like to, most of us know that if we wanted to, we could easily bare our souls to that person and if the relationship is solid, it will only grow with the honesty. However, in a codependent relationship, there is usually a great amount of insecurity as to one's self-worth in the relationship(for at least one member) and as a result, there is a great amount of fear at losing them. This is why being portrayed or coming across as weak in front of your partner can be so frightening. But to avoid this, a person in a codependent relationship is willing to lie and won't be up front so as to avoid angering their partner or receiving their rejection. This is true whether it is a warranted fear or not.
In a healthy relationship, a practice of honesty should always be instituted. Life is difficult, mistakes happen, and problems will arise, and a lot of the time, we cannot deal with them on our own. And it is for this reason that being in a healthy relationship can be such a blessing because you do have that individual to help you. However, if you are scared to even be honest with your partner about how you feel or the things you have done, then you are setting a very poor foundation for a long term relationship, or at least, a healthy one.