1. Nature or nurture?
Not "or" but "and".
Because were determined by our genes, since the likelihood of certain behavioural patterns has proved to be genetically determined.
Because were determined by our experiences, otherwise identical twins would always be exactly the same with exactly the same preferences.
2. Should you choose a college major based on getting a well-paying job or based on what you want to learn about?
Chose something you like where you can make a living of.
There's no need to chose something that will make you incredibly wealthy. Getting by is good enough. But a job is something you'll be doing every day, so you have to make sure you like what you're doing. Choosing a study based on what you want to learn is great, but if there are no jobs in it, you'll face a problem later on.
I personally first chose a study that was about what I wanted to know, but I quit it because it was more of a hobby rather than something I wanted to be my profession. I started a new study later on that was more balanced with what I wanted to know, how much I enjoyed studying, my job opportunities in the future and how much I thought I'd like those potential future jobs.