When you were introduced to the character of Horace Slughorn, you didn't think much about him besides the fact that he unintentionally gave Tom Riddle the idea to create Horcruxes and that he is able to resemble a couch. But what is his darkest secret? Well, this theory from the SuperCarlinBrothers gives you a plausible explanation.
Here We Go
So, this is a quote that Slughorn says during his Potions class:
"When you have seen as much life as I have, you know that Amortentia is the most dangerous potion in the room."
But why would it be dangerous? I mean, you're just getting someone to fall in love with you. I mean, there are a lot of people who would love to give Amortentia to someone.
And besides, the other three potions would give you relentless luck, one that would transform you into someone else, and one that would force you to tell the truth no matter what. If those are the other three potions in the room, why would a simple love potion be so dangerous?
Well, here's something you may have forgotten. Merope Gaunt became infatuated with a Muggle named Tom Riddle. She gave him a Love Potion so that he would fall in love with her, and as a result, they had a child who grew up to be Lord Voldemort.
Well, here's the question: How did Merope get a Love Potion in the first place? It was said that she had little magical talent and her father would call her a "disgusting little Squib." So, how could she have made a love potion? Amortentia doesn't seem exactly like an easy potion to make, either. Who, then, could have made a love potion for her?
One of the important things to think about in theories is timing. Merope was born in 1907, so that means she attended Hogwarts from 1918 to 1925. It is unknown whether or not attended Hogwarts, but we can assume that she did because — as she was a descendant of Salazar Slytherin — it wouldn't make sense for her to not show up at Hogwarts. As she was a descendant of Slytherin, Gaunt would have had a well-known name, and Slughorn is known for his respect famous wizards.
According to the Harry Potter Wiki, Slughorn was born sometime between 1882 and 1914. So, it's possible that they were both students at Hogwarts at the same time. And Slughorn may have taken an interest in her because she's one of the remaining heirs of Salazar Slytherin.
Since Merope had such a distinctive background, it is easy to assume that Slughorn would at least help her brew a love potion, and he seems like the guy who could do it, as he does end up as a Potions master.
This is why Slughorn knows that Amortentia is so dangerous — because he accidentally brewed the potion that conceived Lord Voldemort.