Why did Jeremy have to go into farming? He didn't - he was an adult and a free person. It wasn't the business of his relatives what he went into. He might have liked farming, or he might not have liked it. Perhaps he liked some aspects of it and not others. Does it matter?
Yes, one would have liked to believe that was true. For most, that is the case, but you need to realize that for SOME adopted children, it isn't so. Parents often have plans for their children. They want them to be popular, bright, have successful careers, but usually they haven't mapped out their lives in the way that some adoptive parents do. I don't believe Jeremy was ever asked what he'd like to do, it was probably just assumed he'd do it. He MAY, at some time, have gone along with it just to please his father. Being in such a position -having no choice- is hardly conducive to him 'loving' farming. I'd have said quite the reverse. Then of course, there's the emotional blackmail part, ie the bit about them spending time and money preparing him for the future that was planned for him, and the wealth which would be his one day if he towed the line. All in all, whilst he may have seen an open door -after all, his sister had walked through it and escaped to freedom- if he took the same step, the wealth he'd been promised would be sadly depleted.
I'm not saying that believing oneself to be in such a position is easy for any child, but I suspect it may be more difficult for an adopted child who'd been imbued with the feeling that they owed their parents their gratitude for what had been done for them.