Employee gets a $300K job offer, considers not taking it because he'd have to move away from his fiancée, who's in school: '3 years of long distance feels like a lot.'

That's a life-changing amount of money. 

If you have a chance to make a lot of money early in your career, there's a better chance you're going to take it, even if the conditions aren't ideal. Young people in finance work from early in the morning to late at night because they are less bogged down by family obligations. They are motivated to "make it," and are willing to sacrifice their material desires for a couple of years in order to build a more stable financial future for themselves. 

The thing about working hard and forgoing any play is that it isn't sustainable for your whole life. Eventually, making money is not enough, and you start to get burnt out on making money for money's sake. What's the point of getting rich if you can't use that to start a family or finance an early retirement where you can do whatever you want? If you save enough when you're young and have a couple of miserable years without much material pleasure, you can set yourself up for a much more secure future. If you spend all of your money thinking that it will keep rolling in indefinitely, then all of that grinding might not be worth it.

The question is, how much are you willing to sacrifice, and for how long? Can you deal with not having a romantic partner or many friends for a couple of years to stack cash? Are those sacrifices that you have to make? It's hard to know for certain whether putting your relationships in jeopardy is worth it in the long run.