Some flats are random bad luck. However, there are many factors that increase the probability of getting flats, including:

- Running tires with too low pressure, which leads to pinch flats if you hit bumps, potholes, etc. I pump my tires before every ride to help avoid those.
- Riding in the "crud line" of gravel, glass, etc. that accumulates on the edge of roads and at intersections. This, I am convinced, is one of the main reasons why some cyclists get a lot of flats. They are afraid to take the lane, hug the edge of the road, and pay dearly for it.
- Fragile tires. Some tires just get a lot more flats. They are made for racing, speed and light weight. Tires like Vittoria Corsa CXs or other racing versions. I don't buy these kind of tires.
- Riding on wet roads. Unfortunately, this is hard to avoid if you commute, but it is a well-know phenomenum that you get more flats when it's raining or wet roads because glass will stick to your tires easier and debris gets washed into the roadways.
- Reinstalling tires that flat without thoroughly examining them for glass, wire, rocks that are stuck in the tread. This is the leading cause of repeated flats on the same wheel. If you keep getting flats on the same tire, turn it inside out and closely examine the inside of the tread. Run your fingers along it. Examine the outer tread real closely. Chances are very likely that something is stuck in the tread. Sometimes it's a small piece of wire or glass that is barely visible.
- Old tubes with valves that are wearing out. This is probably the most common cause of flats for me because I repair my flatted tubes and sometimes use them for years. Eventually the valves get worn out and they can't be repaired.

BTW, I don't use heavy thick tires like Schwalb Marathons. I use reasonably light but durable tires like Continental GP 4000s, Michelin Pro Races, Vittoria Rubinos. I get several flats a year but I also ride 7,000-8,000+ miles a year. I enjoy riding too much to use heavy, poor handling tires with high rolling resistance. I would much rather fix a few flats than have to slog up all the hills on heavy tires.