From my experience and research on our C30 smile:
1) Age and hull flexing.
2) Keep the keel bolts torqued to minimize it. (Perform this when the boat is on the hard)
3) Check the bolt torque. Remove any loose filler from the smile. Refill with epoxy and fair. Apply several layers of glass with epoxy resin and fair into the hull.
4) Generally cosmetic, assuming the boat has never had a hard grounding

You might also want to loosen the first couple of keel bolts (the nuts on the bolts) and apply some boat life caulk around the bolt threads and then torque nuts if indeed you are leaking some anitfreeze from the bilge to the exterior of the hull. Make sure the boat utilized washers on the keel bolts, and the nuts do not exhibit signs of excessive torque to the point where they have compressed/depressed the bilge floor. 

By the way- look at other boat brands brands with some age on them and bolted-on keel designs. The smile is not unique to Catalina.
 

There has been many posts and comments regarding the Catalina smile and how to repair it - but I have not seen any possible cuses.
Here is what I believe is the cause - and I have a solution.
There is a keel bolt hidden under the V-birth floor board. To see this bolt remove the floor board retaining screws. The rain water floods the bilge that we see and can empty - but not the one beneath the floor board. Over time this water eventually find its way around the broken sealant of the keel bolt and enters into the plywood below weakening it and the keel drops down. To prevent this I placed a plastic cylinderical enclosure around the bolt and the washer and used 3M 4200 to keep the water out. See images here http://mei1970.org/saillavie/c36Album/index.php?album=Catalina-Smile
The plastic "cup" is the cover of a liquid laundry detergent. The reason I picked this cup was because it is double walled, about 1/4" wide, where it meets the bilge and its wide area provides good space for caulking. The image shows how the water accumulates around the outside of the cup - and keeps the bolt dry.
The other cause is over-tightening the standing rigging specially the forestay. This will bend the bow up and peel the keel away from the hull.

I do not have the smile so I have not attempted to tighten the nut. I hope my solution will avoid the problem.