There is no such thing as being completely honest. (sometimes with yourself and with others)
In children complete honesty means that the end of imagination. No honey, there are no unicorns, mermaids, or fairies. There is no Santa Clause. Childhood is full of lies that help keep childhood fun and pleasant.
In a marriage, complete honesty can be a disaster as well. People are emotional and often irrational. This moment it may be very true that I am not happy with you because of stress that I am feeling over things that do not matter much like chores, finances, or work. It is absolutely true that I may be overwhelmed and tired and your failure to fully notice is angering me but these feelings are temporary and prove to be false once the stress has lessened a bit.
When I choose to share this doubt with you, I risk creating a problem between us that would never have been there. Why? Because we are human and doubt often creeps in?