I have known many people that have landed gear up, and not one of them just forgot to put the gear down. Nearly all of them were in a landing cinfiguration with gear down, and were interrupted in some way that caused them to initiate a go-around and pull gear up. For example, ATC direction, or being cut off in traffic pattern by another aircraft. Then, the problem suddenly clears, and you are again clear to land, such as ATC saying runway now clear, you are clear to land, or other aircraft saying I will go around, you go ahead and land. Now you are trying to recover into landing configuration from an abnormal situation. You may be on short final, and not even remember you pulled the gear up. And then it happens... your first gear up landing.
Yes, I know there is the GUMPF check, gear lights and horns, and all of those can disappear in the heat of the moment. Trust me, they can all go away as you might be focused on making that landing for a multitude of reasons. Notice the recurring theme, interruptions and distractions... always waiting to catch you!
For this reason, one of my personal rules that will not be violated (unless I am on fire or something like that which is more serious than landing gear up) is this:
RULE #3141 If I decide to make a go-around, I pull the nose up as I add full climb power, and when I pull the gear handle, I announce "go around" even if I am alone in the cockpit. From this point, I will not attempt to revert to landing configuration without first going back up to pattern altitude and performing the entire procedure over again.
I have had ATC and instructors unhappy with this rule. On the other hand, I have never forgotton to put my gear down either.