I've been building wheels for just over 20 years now, and successfully passed through that awkward "braking spokes" period a good 17 years or so back. In my experience, if a spoke breaks at the spoke head it's because the spoke head moved around in the hub flange hole. The other kind people posting here have covered the most typical cause of spoke failure, which is the spoke being too loose, but one other possible cause being if the spoke was too thin for the hub flange in the first place (for example Shimano hubs used to use a 2.4mm spoke hole so it was best to use a 2.0mm spoke [14g], whereas Campy hubs used a 2.0mm hole so a 1.8mm spoke [15g] worked best). This can also allow the spoke head to move back and forth in the hub flange hole, which stresses and breaks the spoke at the spoke head.

The OP mentioned that the wheel was recently rebuilt, and this made me think of another possible but often overlooked cause and that is if the hub was built using different pattern. When a hub is built and used with the spokes in one pattern (i.e. a mirror pattern) and is then rebuilt using the opposite patter (switching to a parallel pattern, in this example), the spokes on one flange will switch direction. If the outside spokes originally swept backward, they will now sweep forward (or vice versa...you get the idea). The problem arises because a spoke will commonly groove the hole in the hub, thereby widening the hole ever so slightly. When the hub is rebuilt with the spoke going the other way, the spoke hole gets widened even more, this time by being grooved in the opposite direction. The result is a spoke hole larger than it originally was, and a spoke head that is allowed to move around in the spoke hole. In my experience you can get away with switching from three cross to two cross with the spokes sweeping the same way, but that's about it for changes in pattern. This was pointed out to me by a very experienced wheel builder, and after I started heeding his advice my spoke breaking days were over.

To tell if the lacing pattern has changed look at the hub flange. If you see any marks from a spoke where there isn't a spoke now then the pattern has been changed. If you're looking at a spoke head but you see marks from a spoke next to it, or if a head down spoke sweeps one way but there's a mark from where it used to sweep the other way, then you may have a problem.

One other note: If you have a nice hub that's been built multiple directions and you want to save it, DT Swiss used to make these little brass washers to go over the spoke head to fill in the excess void space in the spoke hole. You can check with DT or other spoke makers to see if they're still available.

So, in summary: loose spokes break at the head. You want spokes to be tight yet even in tension and built on a hub using the same pattern as the hub was originally built. (By the way, excessively tight spokes don't tend to break at the head, but they can cause the rim to crack.)

Just a sampling of my success with my own wheels: I have one set I built in 2000, three rear wheels I built in 1998 (found some nice hubs at a bike swap), and even have a front wheel that I built in 1991, all road wheels. The 1991 front and one of the 1998 rear wheels I use pretty much every day (8 speed Dura Ace hubs on MAVIC Open 4 rims, how's that for a flashback!). I built the wheels on my touring bike, my mountain bike, my commuter, and most of the wheels on my wife's fleet of bikes. I've long since lost track of how many wheels I built for customers in the ten years or so that I worked as a bike mechanic. For comparison, I'm just over 200 lbs and ride between 5000 and 7000 miles per year.

As for the OP's concerns about whether the bike/wheel is appropriate for the demands of commuting: I've seen far heavier riders use 32h wheels successfully for years, but those were well made wheels built with good quality components. As for rough rural roads with lots of pot holes being the problem, those are the sort of things that will damage a rim but if the rims is okay then the spokes should be holding up (in my experience). Now if a rim is bent for any reason (such as from a pot hole) and the wheel is being forced to be true and round, the result can be uneven spoke tension, and that could cause spokes to break.

Hope this was somewhat helpful. Don't be shy about sending a PM if you have any questions that I might be able to help with. Good luck.