not an expert but, at about 6 weeks, most babies have a growth spurt and need more milk. If you're FFing, you just put more in the bottle, no extra stress, if you are bfing, your body takes a couple of days to 'catch up' - so your DC will be demanding more feeds as they aren't getting all they want from each feed until you start making a bit more.
For a lot of woman, this seems rather hellish, draining, and believe they don't have 'enough' milk for their DC, or that their milk is drying up. So stop then.
No shame in it, it's a really, really horrid time and some woman find it harder than others. I nearly stopped, but as i knew DS was due a growth spurt, I set myself a target of week 8, if I still hated it I'd stop then, and by week 8, things had settled down to managable again.
I was under the impression though that the first 3-4 weeks BF was the most important as that's when their tummies are the most sensitive, but that could just be one of those bobbins things you hear about BFing. (Hoping someone with a bit of training turns up to answer question properly)