I'd like to hear more about why you make your own yoghurt. Does it taste very different than store bought plain yoghurt, do you find the DIY aspect appealing, or is it mainly for cost savings?
I personally do it because I get a better consistency and taste, with lower lactose than in commercial varieties, and save a couple of bucks. It's really easy to make, I use a teaspoon of the previous yogurt batch as a starter and let it ferment 13 hours in 1L of milk, in the oven with the oven light on. Too much starter and it ferments too quickly, gets chunky and sour.
I do it to make full-fat, low-sugar yogurt, which is harder than it should be to find at the grocery store. Almost all of them besides the Fage Greek are loaded with sugar. Guess which one our store doesn't stock? (the Fage 5%)
In Spain you can find everywhere "natural yogur", not sweetened, with "4 g sugars", I guess lactose, per 100 g, so 5 g per 125 g cup. Is this low-sugar by your standards or is it possible to achieve lower contents?
Yes, I'm referring to "no sugar added" really. As you mentioned, there is some natural sugar.
Most yogurts here contain various additives for firmness, which usually means some kind of processed sugar, which I prefer to avoid. There is also the quality and the price aspect.
Also, it's fun.
You can clone any expensive yogurt for small price and control the ingredients fully.