I don't recall having ever gone around whilst onboard... however I've instructed a few to do it!
Having skim-read the replies so far the reasons/numbers stack up about right:
The main reason for a go-around (at LHR) is the previous arrival not having vacated in time - being so capacity constrained we have to get them as close as we legally can, and there will be many times where a second literally makes all the difference. It's a 100% safe and standard procedure; If I instruct or an aircraft reports going around I know what they are going to do in terms of climbing/turning without any communication between us. This hopefully allows the flight deck to concentrate on the increased workload for that short period of time.
At the moment there are taxiway works which necessitate the closure of the main short-haul type aircraft exit when using 09L (approach over Windsor). This means that everyone has to roll a few 100m further down to vacate - nearly all do it quickly and efficiently - and we have had to widen out the gaps inbound to allow for this, but sometimes it just doesn't work.
Other reasons can be - windshear, turbulence, crossing traffic not clear, debris, lights, weather, emergency traffic, animals... the list goes on.
One or two go-arounds a day is about right - I recall one day in strong wind of 43 go-arounds a few years back. The tower was swaying a bit...