Consider how we think of the self spatially. Much of the time our awareness is on the front surface of our bodies or just in front of that. We are thinking about external events or tasks, and it is necessary to look into our hearts to know what we feel. In this phrase, the heart is not entirely metaphorical: the area around the heart is, along with the belly, throat and facial muscles, a place we direct our attention in order to identify our feelings.
Sometimes, however, we need even deeper insight into ourselves because there are additional feelings or motivations that are, as it were, behind our superficial reactions. Our motivations and needs are so far toward "the back of our mind" that they seem to be in a special metaphorical space called the unconscious. (In the MetaSelf model, the box-frame, this space is behind the backboard, which casts a shadow onto the wall. This shadow represents the repressed needs, feelings and potentials of the self, which have, as it were, been pressed to the back of the figurative space the self occupies.)
All this helps us make sense of the sentence. "I'll be upfront with you." If we want to hide something from people, we may stand in front of it; if we want to hide it from ourselves, we can "put it behind us." "Out of sight, out of mind." By contrast, in order to deal with something courageously and honestly, we need to look at it and face it, con-front it.
The front of something is usually the side that interacts with the world. In our own case, the face is where most awareness is -- eyes, nose, mouth; and, although the ears are on the side of the head, we use them to locate things and face toward them much more often than away from them. The face is the side we show to people we accept and want to communicate with fully. When we are "upfront," we are honest about the deeper things we feel, we "put them out there," or, in visual terms, we are figuratively "transparent." Transparency was used by the psychologist Carl Rogers as a metaphor for honesty; a person reveals himself or herself in order to produce intimacy and build trust. (The box- frame model has a transparent front space for the conscious part of the self.)
On the other hand, we use "put up a good front," suggesting a false front, a "mask," or what Carl Jung called the "persona." (In the box-frame model, this mask is represented by a coat of paint on the backboard.)
"Out front" is different from "upfront." It suggests being ahead in a race. It describes comparative position relative to other (moving) objects. Thus (in most instances in English) the leading side of a perfectly round moving object would be called its front, despite the lack of distinguishing features and even if the object is moving away from us. On the other hand, if the angle of the object's figurative view of us is being spoken of, then "in front of it" will mean between us and the object, regardless of its motion.
The connection between "upfront" and "out front" stems from the fact that the front/back axis of our body is linked with our mode of locomotion. (Fidler crabs, with their one big claw and their sideward crawling, are different from us in this regard. They lack bilateral symetry.)
In English, the prefixes pro and re call our attention to the contrast between forward and backward motion. Forward motion often implies a goal, so that forward implies something desirable while backward implies either a loss or a strategic retreat. Progress and regress, progression and regression. We have phrases like "the leading edge," and "bringing up the rear." Being "forward" or "pushy" socially, and "making advances." Being "backward," meaning socially or mentally retarded.
Someone might say, "I love you deeply. This is not a superficial 'front' I'm
putting on, not a mask." Depth, of course, isn't necessarily downward. It is
measured from the viewer's location, along a general line of sight, so we can
say "a deep hole in the ground," "a deep hole in the wall," or even "a deep
hole in the ceiling." In the MetaSelf model of the self, depth is mostly from
front to back. "Deep inside, I really love you." We are thinking of depth as
being measured from a surface point where consciousness is located, and we
are looking back into ourselves. And we can add vertical depth: "Deep down
inside..." and we speak of pushing our feelings "down" out of awareness, as
"back" out of awareness.