Uhm I believe they're not determined to make him sad. But more likely his family thought that's what's best for him.
The deal about Hakyung being Jiyoung was clear to the family(this includes Yoonsung knowing s/he is a fake). It was unexpected though that Yoonsung would fall for "Jiyoung". When Yoonsung had amnesia, that's when they thought it's best opportunity to let him be totally forget "Jiyoung" person. There are certain events why they had to do this. Actually, I really love the revelations as we go on in the series. Whent I thought it was done, then we'll have one like "Oh it makes sense". Anyway, as human, they tend to make a wrong decisions too added to this is the father's greed. BUT I won't paint him totally black. I find his character a supportive father to "another child" later on. He just can't bear Hakyung because I believe deep down he's totally guilty and can't do anything about his son.
We should not forget Hakyung and the father is in agreement here. It's not like Hakyung has the whole disadvantage. This is the very reason though that while Hakyung is a good person, still he's one of those who will do anything for his family, just like them doing what's best for Yoonsung.
he must feel so ALONE. It makes me feel DEPRESSED. He’s such a good guy but no one is there for him.
Yoonsung deserves all the happiness in the world. My baby has loved everybody unconditionally.. So why is everyone determined to make him sad always ╥﹏╥