Frost is indicative of an air leak (which means that the door seal is messed up).
Pardon me while I jump off into a thermodynamic primer here...
the "cold" is made in the freezer compartment. There's a fan in there
that circulates this chilled air around... Only a portion
of the chilled air is
directed (ducted) to the fridge (beer) section.
Here's where the nifty thermodynamic whiz-bangs happen... Hot air balloons float why? Because they're lighter than the air surrounding it! (And a hot air balloon is "attracted" [like a magnet] to the coldest spot in the atmosphere) Ya know why? Because nature abhors a vacuum. Nature is always in a struggle to balance itself. So the cold air above actually pulls
the balloon into the air and tries to stabilize its' temperature and equalize it with the surrounding environment (at that equalization moment, you wouldn't wanna be in the basket... know what I mean? Crash, Boom, Bang)
By "refrigerating", we are, by nature's rules, "breaking the law". We're actually pulling
(removing) heat (calories) out of the food and transferring that heat to the outside of the fridge. Nifty, huh? Instead of applying
"cold", we actually are removing heat
Ok, OK... back to your issue. Frost is only water vapor that is suspended in air... When this warm air comes in contact with a "cold" surface it condenses OUT of the warm air and clings to the colder surface... think of a glass of iced tea on the patio, you know how the glass "sweats"? Imagine that your glass of tea is 22 degrees below zero Fahrenheit and now imagine what would happen to all the warm moisture laden air surrounding it. Yep, you're right... It would sweat out on the coldest surface and then quickly freeze/frost.
Now back to the balloon thing... (and nature). .. Cold air is "heavier" (more dense) than hot air, right? So if there's a leak (think of air as a fluid) of cold air [which flows toward the floor] there MUST be a replacement, right? (The whole "nature abhoring a vacuum" thing) Cold air flows out and warm (moisture laden) air fills the vacuum. Where does this warm air go?
Yep! It goes where the balloon goes! To the coldest spot it can find! (You're good at this!) Where's the coldest spot in the fridge, you ask? Well? It's at the top left, inside the cabinet. In the very back, you'll see the damper. It's used to adjust the amount of cold air flowing into the fridge compartment. When warm air hits this (the coldest part of the fridge) it immediately condenses and frosts over. This frost builds up a wall. This wall stops air from circulating. When air stops getting to the fridge, your eggs spoil, your mayonaisse stinks and your beer isn't worth squat.
Now for the "WHY" of it all... Could be several things allowing warm air into the fridge.
First thing to check is the rubber door gasket. Make sure it's seating properly all the way around the door. If it's not, the door may be warped... try to warp it back into shape. You have to do this physically.
Second, use some "Vaseline" and rub a thin layer of it onto the door gasket (the part that seals against the frame). Not only will this help make a good air-tight seal, it'll also help keep the door gasket supple and intact for the life of the refrigerator.
Take this knowledge and prosper... (only AFTER you've rated this soltion as "FixYa", though)