Difference between toxic and fluffy yaoi

Apart from those with the really toxic couples that makes you so angry while reading, there are some manhwas that I think are really cute but majority don't see them as cute. Like third ending, i love that story re-read it countless times and to me its a really cute and cuddly story that I go back to when everything is boring, but I noticed a lot of people find the story really toxic and I honestly don't see why. The amount of sugar there can cause toothache so why do y'all think it's toxic. My brother's lover is toxic, third ending is cute.